The smaller skeleton would point out that he's smooched and even slept in the same bed as Toriel before (which I had to word really carefully just now, wow), but here comes Mettaton. Literally.

Which pretty much confirms what Sans thought MTT looked like, though he only gets a quick glimpse of it before he's hiding again. Sighing, he gives a casual shrug, his usual lazy grin, and spins around in his chair.

"I ain't looking. Give it a try."