a) As you board, there is often a plate inside and towards the top of door frame, especially on Boeing narror bodies. This has the manufacturers construction number on it.
b) If the flight deck door is open as you board, look for the registration on the flight deck instrument panel - its always there.
c) On narrow bodies, as you board you may see that there's often a paper certificate of registration on the panel to the left of the forward door or on the back of the flight deck door.
d) Ask the cabin crew for the registration or the fleet number - if they don't have it on their paperwork, they'll usually buzz the flight deck to ask.
e) As a very last resort, wait til you get off but realise that there's no guarantee you'll be able to see.