I think you can tell Fuller is back because suddenly Sylar can do stuff again that we have never seen him do before and do not know how he can do it. We saw this a lot in season one as well. Remember in season one when he was being shot and some how healing without having taken Clair's power, falling off a roof, yet getting up and walking away yet Peter who feel the same distance was all bent in half and dead? Or how about him flying away without having the flight power? Yep, looks like season one Sylar is back.