Is it legal to push back the opponent while posting

[Amesman]

Been itching to ask for views on this scenario for a while and this seems like a good opening ...

Two-whistle game, you're lead, administering B1's free throws. During said throws, what's your best advice for determining whether A2 (in the lowest position, opposite) is getting pushed by B2 to the point of a foul during the shot and boxout?

Let me clarify: A2 is not jumping, but rather digging in for the box-out. There's a total straight-lining there.

I knew the family of A2 in one particular game and asked later if he was really getting the pressure from behind and he said yes. I had thought as much, yet from the endline, I saw no displacement so figured I had nothing to call. I told him it would have helped if I saw him move or lurch at some point. Thoughts?