She's just about forgotten how to breath. And stand upright; her hands still, pressed against the flat of his back as if she's convinced she'll fall over if he moves away. Her scent is definitely noticable, which he knows is a sure sign she's turned on, and her heartbeat just outsped Bart.

Or near enough.

But if all that isn't enough, the full whimper and accompanying moan should signal he really is doing everything right so far.