Flying Europe

if they check then that's where it would be.

I can't recall that it has ever happened to me, but I was always prepared if they did ask (re. weight, dimensions).

Usually at the boarding gate, but if you check in at a manual desk and not online or on a machine, they will check there as well. They usually have baskets in the check-in area for you to check your bag is OK.

In my experience it is sometimes at the gate and sometimes at the bag drop but it really depends on the Carrier.
The Low Cost Carriers are more prone to checking sizes and weights .Sometimes at bag drop they will have a quick look at your carryon bag and tag it, if it looks a bit big or heavy they will measure and weigh it there, tag accordingly or suggest you re-organise your bags.
I have never had my bag weighed at the gate but have seen it done but I seldom travel carry on only so usually just have a smaller bag with the important stuff in it and the things I need during the flight.
The important thing with all European carriers is to make sure you conform to their usually strict regulations for carry ons, if they say one bag they mean one bag and not one bag and a personal item.when they say max weight is 10kg then it is 10kg not 10.5 or 11kg.

I agree with Unclegus. I have seen it done at both. Easyjet has two different size allowances, the smaller of which they guarantee you will be able to carry on and a slightly larger carryon that may have to go in the hold if if the flight is busy. The latter will be decided at the gate and I have seen this done, much to the passengers annoyance.

You'd really have to get a bit more specific: Which airline? Lufthansa, Air Berlin and British Airways almost never check your carry-on unless it's abnormally large or so heavy that you appear to be in pain carrying it.

EasyJet, on the other hand, do check size and weight of your carry-on frequently.