i guess the confusing ones would be as follows:
A340 - can be RR or CFM
B744 - can be RR or PW
for the A340 - the CFM engine does not have have an exhaust protruding from rear of nacelle - but the best clue is the size of the engine - the trent 500 is a lot bigger than the CFM
for the B744 - the RB211 has no exhaust/core protruding, the PW has a short reverser, the core of the engine is visible, and it has a large exhaust plug at the rear
hope this helps a little.
regards,
n1 vibes