locate the correct train

Just like at the airport there will be a sign listing the departures but a lot bigger.
it looks like this:

https://www.la-croix.com/Economie/Entreprises/SNCF-audit-pointe-dysfonctionnements-lorigine-retards-2017-07-16-1200863303

The sign has the time the track and train number that will be listed on your ticket. Look for the train number and time.
It has the destination listed as well but it if Bayeux is not the last stop of the train you will have to look for Bayeux in the list of all the stops as those will all be displayed sometimes scrolled from right to left.

We took the same train trip on our first trip to France. I worried about how we’d identify the right train but it was easy to read the boards. We bought the tickets at the station and they were valid for any departure time that day. Don’t forget to validate your tickets using the machine at trackside before you board! Failure to do so can result in a big fine. We didn’t know this, didn’t validate our tickets, and were in the process of being charged a fine by the man checking tickets but the local couple in our train compartment talked the man into waiving the fee for us--Americans who were first time tourists in France and didn’t know the rules. Ignorance is no excuse but he cut us a break.

great advice-thank you so much

The train planner of the SCNF (or the Bahn.de) gives you also the final destination of your train, which is the information most prominently displayed on the departure boards.

The end of the line for route is Cherbourg.

The electronic sign board similar to this will have the train information and the departure track (the word you're looking for is "Voie" from what I remember).

You may also find this helpful - https://www.ricksteves.com/travel-tips/transportation/trains/getting-on-the-right-train .

If you're connected whilst in France (and I'm the only person who these days doesn't seem to be) you can use this
https://www.gares-sncf.com/fr/gare/frpsl/paris-saint-lazare

which train would be the best train to Bayeux from St. Lazare RER or OUISNCF and which is the best way to purchase tickets?

Hi,
Only SNCF trains go to Bayeux. Either:
1. Purchase tickets in advance, no earlier than four hours after your scheduled landing time at CDG, using either the Oui.sncf website/app or the Loco2 website/app (Loco2 is a reputable agency which has a better record of accepting US cards).
Or:
2. Buy tickets once in Paris or on your way to Paris, either using the aforementioned apps or the ticket machines at Gare Saint Lazare.

I would recommend option 1, because you get a better price if you buy in advance and you are still able to cancel/modify tickets (for a fee) if you are late (as long as you do so more than 30 minutes before departure).

great advice as usual. Another question best place to exchange dollars for Euros?

Don’t exchange currency anywhere, that’s the best answer. Take your atm card and get local currency at atms all over Europe.

have to pay cash in some places like Overlord Tours as my cc (capital one) does not work with their payment site.

if we pre purchase OUISNCF in advance as you mentioned are you obligated to that train time or can you use it for later trains and once getting to the station is there an easy way to find OUISNCF?

and once tix are purchased do they have to be printed or can you download to your phone?

hey hey sruniform
I’m a little late with your dad rip and some suggestions but some options to check out. Did you try loco2.com with your credit card?
a few things in Paris:
canauxrama.com. A few different cruises around Paris and further out, maybe countryside down the seine or one that stops for lunch at a stop on shore
o-chateau.com has a champagne cruise
auberge la goelette in andresy. we are taking short train ride from Paris, cross small bridge and meet at little boat dock, ring bell and boat comes to get you for lunch at outside cafe on river
cite floral look it up, beautiful flower garden for your wife, (frenchmoments.eu search village of cite florale)
rue cremieux. Near gard de Lyon. a cute cobblestone neighborhood with pastel colored houses with flower boxes.
discoverwalks.com
search “best of Paris on a unique ride” with julian. no tired feet
we will be there for 8 days end of September in the 14th arr. can’t wait, our third time and in a totally different area. thanks to janettravels and trocard for the comments about the area, away from the hustle, bustle, and “zoo”
enjoy your vacation and don’t forget to stop at cafe with glass of vino, people watch.
aloha

Yes, usually if you get an advance ticket you will be limited to that specific train. As far as finding the train goes, people up above have already told you in detail - all trains above ground at St Lazare are operated by SNCF, it's the single national rail company.

Advance tickets come with a seat reservation for a specific train. Often the train platform will have a diagram posted showing where to stand for boarding.

Over-the-counter tickets for some destinations required a stamp from a small machine at the entrance to the platform, indicating that the ticket has been used. Print-at-home tickets don't because they are dated. The ticket can be shown to the conductor after the ride started. However I was in Gare de Lyon recently and saw that the long-distance trains now have security gates where the tickets are scanned like in an airport.

The blog Paris by Train contains photo guides to some of the stations, helpful in orienting yourself:

https://parisbytrain.com/gare-saint-lazare-train-station-photo-guide/

Thinking of things to add? As I recall the trains you want are on the last few tracks (want to say North end of the station, but I'm not sure it's a N/S configuration?). There are roughly 15 tracks. It sounds like a lot but it's really not too hard. If you get lost or confused, there are people dressed like conductors (only not on trains) you can ask for help.

If you arrive at the station more than 20 minutes early, don't freak out if you don't see your train. Trains are typically posted only 15-20 minutes before departure.

I always pick my ticket up at the station kiosks. They're like airport kiosks to print boarding passes. They say you need to have the same credit card you bought the ticket with. In my experience using the credit card is an option. If you know your name and ticket code (just like a plane ticket), you don't need the credit card.

Most tickets need to be validated. If your tickets says something like "good from ___ hour..." at the top, it's an open ticket. There are places to validate at the end of the platform before you board. Watch other passengers to see how they do it. You just put your ticket in to stamp it. If you really can't figure it out, ask a conductor before they come around for tickets.

I'm guessing your train will be about 3 to 3 1/2 hours. Stop at one of the shops and buy a sandwich and drink for the trip.

Getting cash: There are a lot of Euronet machines nowadays. These are currency exchange machines and aren't the best option (they add a charge in the form of an unfavorable conversion rate). Use a cash machine with a bank name on it. I prefer the ones that let you pick the currency (I much prefer having ten euro bills to larger bills).

Credit card: I've had many times when French (not German, Spanish, Danish, Swedish, English, Irish or other) machines have difficulty processing my credit cards. It's been true at toll booths, metro machines, online or any other automated option. I've found the first thing to try is use the same card a second time. My theory is your transaction times out before it gets approval from your home bank. For some reason a second try seems to work as often as not?

Best train: There are multiple trains to Bayeux. The best is any direct train - meaning you don't have to change. They will take less time and less hassle. Search for trains at SNCF or Oui.sncf. That's the French rail site. I've used it many times. It works.

When you search, look for duration of the trip, the shorter the better. The site shows a handful of options around the time you searched. Be sure to search earlier/later to find the best options.

Advance tickets ("Prem's" on OUI SNCF) are tied to a train and cannot be used in the next train BUT they can now be exchanged before departure, or refunded up to 30 min before departure, for a maximum fee of 12 euros (or 50% of the ticket cost if that's less). This is a new fare rule as of May 2019, and the reason why I now recommend advance booking.