There is no place where UA states exactly when they will CPU you. You also have no way of knowing if you are "entitled" to that seat since you have no idea where you are on the list. I don't believe the CSR can see the list either. Unless there is an issue with your reservation (no upgrade eligible/requested or out-of-sync), there's no point in calling in.

Now if you're already on board, are at the top of the UG list and see empty F seats, you may want to inquire.