One "trick" that I like to use when I see "signs" like that (urine smeared around, blood trails on the paper, etc) is to pop the male ... If they have been copulating, usually one pene will come out a little easier than the other and will be slightly darker and sometimes even more swollen (a sign of recent use). Often you'll see one pene with a sperm plug and the darker more swollen one without. For me, that's a definite confirmation of a lock.

These are the types of little tricks I love to hear!