You can type with an accent. Brits have various vocabulary, spelling, and idiom differences that might give them away (although to be fair, I probably wouldn't recognize a specifically British idiom if I saw it).

There are two points here: (1), I don't interact with very many people on the internet. I certainly don't use it to meet new people. And (2), I don't look for content aimed at (e.g.) an English audience. I read a large number of different bloggers, but it seems to have worked out that they're American. (John Derbyshire is English, but has lived in the US for years, and as far as I know has no intention to leave. Does he count?)