Most people have mannerisms or some identifying marks when they talk and especially type - typos, grammar mistakes, sentence/paragraph structure, specific slang or usual words they use. I've spotted quite a few alts (or suspected ones) over the years just by the way they type. mdayonliner fairly recently created a new account that he purposely wanted to keep separate and anonymous from his original and within a few days I had an inkling that it belonged to him. He then referred to me by a variation of my name in a specific way that I had only ever seen him use that gave the game away fully. A few other people have since noticed it.

(especially if they're still active on another account or elsewhere... would be hilarious if satoshi was still on the forum trolling on another account haha. Maybe he's one of the fud trolls)