I'll answer this question properly, rather than flaming.
Wild generalization number one: Seeing as 'America' was primarily formed from colonialists out of England/Ireland there are no visible differences between them and those here in the UK (the rest of Europe is a slightly different matter.) So the differences are cultural:
-Americans in London will often have a map, a fanny-pack-thingy and a camera around the neck, with shorts. Ye olde tourist.
-They will talk to you on the tube
-They will talk to each other on the tube.
-They will not have a mobile phone glued to their face
-They will use a map
-They will get lost using the map, and, crucially, ask for directions.
-They will ask for a regular beer in the pub
...and so on. Visibly, racially etc. the world is now so multicultural you can't tell from raw appearance alone who is from the US. But in practice, there are difference abound that give us here in Europe that 6th Sense.
*This thread contains more generalizations than any other on the face (face?) of the internet.*
EDIT: Bubblegum chewing is a surefire sign of US-dom