Yes, that is basically it... just ask if a reservation is required or not.
The general rule is not to worry, though... for the most part you only are required to have reservations for the following trains, all of which are very clearly marked on schedules by their abbreviations:
TGV (in France - it is three euros per reservation)
Eurostar (the one between London, Paris, or Brussels... this is not included in your Eurail pass)
ES (in Italy - the Eurostar Italia is not at all related to the northern "Eurostar" despite the name. Reservation is required and it is something like sixteen euros per reservation... well worth it in my opinion)
Night trains... any night train at all.
ICE Sprinter (Inter-City Express Sprinter - in Germany - these trains are not common. The ICE is the high-speed train of Germany, and you do not need to reserve... the ICE Sprinter is a version that will only go between the biggest cities - here is a list of those routes. You can take an ICE on these routes instead of the Sprinter - it's longer but no reservation required.)
AVE (in Spain - the high-speed trains)
Alright, while this is not an exhaustive list it likely is the only ones that you may regularly encounter. As long as you keep these in mind, just hop-on, hop-off the rest.
By the way, will you be travelling on the Rhine in Germany? If so, your Eurail pass will let you on the KD line boats at no extra charge.
Have a great trip!