iCloud iMessages syncing status is hidden on iOS in b6 and there's no way to force it. Also, it only syncs new messages received or sent after it's been turned on. It does not sync existing messages. This is the way it works in these betas and not sure if it's designed this way or if it will sync old messages when it GA's.
Typically, when Apple beta tests features that are dependent on iCloud, beta users get switched to iCloud beta servers and both iOS and iCloud are being developed throughout the beta cycle.