Cum on toilet seat

Cum on toilet seat is a popular party game and can be considered a sexual prank:

He enters the party restroom and ejaculates onto the toilet ring. After that he carefully wipes his jizz all over the toilet ring, especially at the rear and front portion of the ring. His jizz on the toilet ring will be almost invisible on typical white toilets. The next girl who sits on the toilet will not notice anything until she gets up and notices that the seat is kind of sticky. If she wears a miniskirt you can see a ring of his jizz on the back of her thighs.

Who would know that there is a lot of jizz on the toilet ring??