[physicsref]

I'll probably just ask my P "Did you see the ball knocked loose by the Defense?" Thus giving him the opportunity to change his mind. I don't like the "I have definite knowledge that the ball was knocked loose..." approach. You don't give the guy who made the call a chance to defend his call, you basically insist he call it your way. By coming with a question, you let him know that:

a) You saw something different/additional.
b) You don't agree with his call.
c) You will respect his call.
d) He may reverse it.
e) It's his call to change or not.

Most likely, he will infer from your question the above mentioned points as well as:

a) Hey, I blew that call.
b) I have an "out" now.
c) My P is not trying to show me up.

That said, I generally will not question a travel call, but this sitch is a little different. Namely, when the violation gets called by my partner, its usually in his area or in a grey area of shared coverage or I realize that his view was a good one.

Maybe you pregamed something like "If I'm L and there's a drive in the lane, I'll be watching up high for contact, the travel can come from the outside official..." etc. etc.