Just because you didn't see a lock does not mean it didn't happen.
Before you pair a male, check his penes. They will likely look quite normal if he has not been paired for a few weeks. After pairing- check them again. If one pene is larger and a brighter reddish color than the other, it is likely he locked your female. You will probably see sperm plugs on the pene that looks normal. That is the one he didn't use. I know which males are "left pene", or "right pene" lockers from checking after visual locks.