How do you find out who owns what?

[Torn Rose]

I've seen this a few times here and other places and was wondering how people find out what other domains people own.

For example, some thief stole from Cleo recently, and all they had was one domain listed, but then Cleo post more domains this person owned, how did Cleo find these?

(Not trying to point you out Cleo, just a recent example so I'm more clear in my question)