I'm a moderator on three
Java forums, so I can't help spotting cross postings. And sometimes cross posts have a giveaway: for example, mentioning an image/screenshot on a forum that doesn't allow attachments, or posting unsupported BBCode or HTML.
There's also something about the attitude of a typical cross poster that sometimes makes me want to Google the first couple of sentences, in quotes. About half the time I do get multiple results, usually on forums I don't read regularly.