During the "test" portion of the video, while Julie and Boomer are away and being watched on the TV screen by everybody else, half of Julie's body is clearly visible next to Arthur when the other kids are cheering.
When Sam Sprocket makes the bike helmet appear in his hand, the trees in the background noticeably jump at the moment the helmet appears.
While out on their ride, after Arthur asks "is it just like at a driveway and a stop sign too?" Sam's mouth moves to say something, but no sound comes out.