I don't really assume that people on the internet are British or American. There are some characteristics that allow me to recognize certain nationalities, though:
1) When someone posts how great it is to be able to smoke weed and how great beer is, along with a "

" smiley, I know for a fact that the person is Dutch. And it embarrasses me.
2) Often, people have the country code in their nickname. Names that were quite common in WoW, for example, were names like 'RogueDK' ( or its equivalent 'RougeDK' ), ShadowHunterNL, and other poorly thought of names.
3) When playing Company of Heroes, one can assume that most people are either French or German.
4) When people don't know the difference between your/you're and there/their/they're, they are from lazy-retard-land. It really isn't that hard, but people just can't be arsed to learn how to use these words..
Can't think of anything else right now, but I'm sure others would have something to add to this list.