I Can tell u for a fact I never played on them from around Christmas on. If I were on a dedicated server there would be no host migration. If I were on a dedicated server, in between matches I wouldn't see "finding best host" at the top of the screen. Dead giveaways.
No host migration... for sure that's an indicator but not a certainty. But I don't think "finding best host" is a true indicator...
... as the composition of a room changes, the best host might not be the dedicated server.