kann mir jemand das erkären?

Cringe.

That’s pretty much the dumbest video I’ve seen for a long time. No. A dentist can of course NOT see if you’ve blown someone. Sperm remnants in your mouth are also hardly possible unless you have given your friend a minute before you put on the chair, blow one, and you still have your mouth full. These are mucous membranes and you swallow your spit, so also any residues that are still present in your mouth. But let’s get to the at least halfway true half of this video. If the blowjob was harder, it can come to petechia (small bruises) on the soft palate. A dentist could theoretically conclude that you gave a blowjob. However, this is also only the case if it really went hard and these petechies can also be caused by other circumstances. For example, blood clotting disorders, or the consumption of solid food. So it’s not a clear indication of a blowjob.