try to look down at the nosegear u/c doors for at least the last two letters. Look up at the top of the tail as a tip also. Some airlines dont have these markings in those places but worth asking a crew member. Ask ath the gate as well as A/C reg should be on the manifests/check in screen.
I asked the crew once, on a LUX-LGW B737 BA flight. Rather bolshy crew member thought I was suspicious and "what kind of a question is that". He kept gave me strange looks right throughout the flight.
He told (G-DOCx) me but I fealt guilty about it as if it inconvenienced him.