Why is it so important to prove this point?
What are you hoping for an outcome?
Let’s say such a product exists.. (probably doesn’t)… would that really change the denial?
If you had proof that said person misses the toilet during urination, and they finally admit it… now what?
I understand that this may be disgusting. The denials are probably infuriating.
However, I suspect that you could get Chris Hansen to do a full-blown Dateline NBC investigative piece to solve this “mystery”. What will improve?
It seems to me that the best answer is to change the chore list so that said person is responsible for clean-up of that bathroom. Then, regardless of the reason why, it is either clean, or it is not.