Right!
Ishy said to Lanfear in S2 that he has had some practice in the World of Dreams, after she complimented his abilities. If he was able to Dream for thousands of years (or even hundreds, if it was only cyclical like in the books), he would have had a LOT of time to practice and hone some skills.
We know there is the ability to track things down by focusing on need. It isn't far to extend that to focusing on extreme emotions, channeling, etc. There was a lot we didn't get into even in the books about abilities in the Dreamworld, and since so much of it is tied to will and practice, I don't think it's a stretch that an extremely intelligent Age of Legends Aes Sedai would figure out how to track down channelers, tav'eren, and more in the Dreamworld.