Sherlock can probably tell just from the sound of the way the water is hitting them. Ted tries not to worry about that too much, because it's a lost cause, and it doesn't seem to bother Sherlock, nor Joan.

Ted weaves his fingers into her hair, a little uncertainly at first, but reassured by her hand on his. He knows she doesn't mind mild pain, that she even likes it in some circumstances. And he trusts that she'll let him know if he's doing something she doesn't like, because she knows he would never want to do anything bad to her, even if all she does is just offers some gentle redirection.

This is gentle encouragement though, so he lets his hands tighten a little bit in her hair. It's more of an involuntary reaction to the sensations, than something he intentionally does, but if she likes it, then he's fine with that.

The pleasure is building inside him, and he reaches his other hand out to steady himself against the wall, biting his lip.