No direct way of seeing if your upgrade will clear, but there is an indirect way. When I've done this in the past I've gotten on united and checked every day to see if the PZ fare has opened up on the leg I'm trying to upgrade. In expert mode, do a search on the leg that you're trying to upgrade and look at the "details" of the flight you're booked on to see if PZ > 0, which is used for mileage upgrade awards. If that goes >0 I would call in and check to see if you can be upgraded. There is a waiting list that you can't see, before the one referred to by kevflyer, that will determine if you'll get upgraded on the spot when you call in.
I'm sure others have more experience and can chime in with more details.