I've done this before when I've been straightlined. Probably a few times per season I rely on this mechanic - all other times I hope to be in the correct position.
I have an offender going to the hoop. I see the defender jump from an obviously specific spot, such as the block. The defender then lands 1 foot further away from the hoop. Plus, there is sufficient body contact with the offense. If the defender wasn't pushed from behind, which I would see, then he must have jumped forward because he landed further ahead. This proves that he did not maintain verticality. Then, with the contact, it's a foul.
Anyone else do that?