The guide I use is a bit rough but is as follows: The first giveaway is to look at the end of the fuselage. Look to where it comes to a point on most a/c but on the 777 it ends with a small vertical section which looks a bit odd. The APU exhaust is on the Port side of this ending (on all the 777's i've seen). The next obvious thing is the engines, if you are not the sort of person who can distinguish between types of RB211, then no probs, the engines on a 777 are LARGE, whichever variant. Look at the window size to get a rough idea.
Next is the fuselage in general, it looks a lot bigger than most other aircraft and is easily distinguishable from a 767. Finally, look above the flightdeck, the fuselage seems to curve for an awful long time on the 777 and this looks significantly different from other aircraft.
I hope that I have helped a little with this. If you don't understand it, dont worry, its my poor explanation. Of course you could always cross-check the reg or use a pair of binocs to read the "boeing 777-xxx" off the back of the fuselage.