Because a Whitecloak, of all people, is going to just randomly stumble across a bunch of White Tower Aes Sedai rings. You know, just found them for sale at a street corner market.
I mean, think about it for a couple of seconds, eh? These rings are only given as the mark of a fully trained tower sister. They are the literal equivalent of both military Dog Tags for a Special Forces Unit crossed with an Ambassador's Credentials. Wave an Aes Sedai tower ring infront of any political flunky in most countries and you instantly open a boatload of doors that other people would never even get a chance to see. The Tower would probably have very strict policies about retrieving lost rings, and god help anyone who was found attempting to counterfit one.
There is zero chance a Whitecloak Inquisitor just happens to be wandering around with a personal trophy collection of these things unless he took them off an actual Sister's corpse. There is also zero chance someone like Valda is going to show off a bunch of fakes just to try to make himself look good.
So yea, if Valda is showing off rings, there is basically only one likely conclusion as to how he ended up with them.