I think that one of the things that confuses a lot of people is that pretty much everyone on the planet can relate to the sense of having "different personalities".
Eg. Jack talks/walks/dresses/thinks/behaves differently, like, completely differently depending on whether he's at work talking to his secretary, at home alone with his wife, having dinner with the in-laws, helping out at a motor vehicle accident that occurred just up the road. That's not just helpful, it's totally normal. Home-with-wife Jack would get fired if he didn't switch into Workplace-Jack and would be useless at the scene of an accident if he didn't switch into emergency-Jack. Soooo normal. Soooo not the same as DID.
When of the big tell-tale signs is your memory. If Workplace-Jack only has memories of those times he's been at work, and Home-with-Wife Jack can remember the whole of his relationship, but no idea what goes on at work, that's a sign that the personality states are dissociative, rather than normal adaptive behaviour.
Small sign. But a big big difference.