First class is usually at the London 'end' of a train. There are some exceptions as noted on here, but it's not a bad rule to go by.
The other thing to do is look at the information screen on the platform. At many (not all) stations, particularly for longer-distance trains, this info can be displayed along the 'rolling news' bit of the screen, under the main destination info. Failing that, do ask one of the staff as they'll usually know.
I'd second Raffles' point about seat reservations. The other thing you should know is that even if you have reserved seats, you can take other, unreserved seats onboard with no problem whatsoever. This is useful in the exact circumstance Raffles mentions, when the reserved seats are stuffed into one carriage, with the next (unreserved) carriage nearly empty.