George is taller
- George has a mole on his neck
- George's upper lip has a downward curve in the middle, like a heart. Fred's upper lip is straight. (Personally, I find the way they smile to be the easiest way to tell them apart.)
- Their eyes are different. George's are slightly bulgier. Fred's are darker.
- The tip of Fred's nose point slightly up, George's point down. Also, George has a slight bump on the bridge of his nose.
- Fred has higher cheekbones.
- And obviously, in Deathly Hallows, George is missing an
ear.
The Fred-on-the-left, George-on-the-right thing is not always true, so it's not very reliable, especially in still photographs which may easily be flipped.
If you mean how to tell them apart in the book, usually they'll be named, but if not: Fred tends to be the leader and is more impulsive. He can also be more brash, if not meaner. George is (relatively) calmer, more of a planner, and if any one of them expresses doubts about a given plan it'll usually be George.
