The ones I have found hardest to distinguish are the 767-300/400s from the 777s, but i found you can tell them apart easily by looking at the engines and the tails. On a 777, the tail extends a bit farther out beyond the vertical stabilizer and has a distinctly squared-off end. Also, the engines of a 777 have a very short exhaust cone and a very large nacelle (main engine cover).
I beleive that the only way to tell apart the A319s and A320s is to know that the A319 is 13ft. shorter.
If by "756" you mean 757, then i can tell you that a 757 looks like an long, skinny version of the 767 (although it is actually 4ft. shorter than the 767).