When to use Miles vs Certs for Upgrades
 

How do you determine when to use miles versus certificates for upgrades? I know that I can confirm space at time of reservation with miles which is nice, but some times I want to use my certs that are starting to pile up. Is there any way to check and see what my chance of upgrading are before hand? I know the bigger planes give us greater chance and I may be able to change routing to take advantage of those planes. I get a feel when I travel to the same spot, but not when I go to unusual spots like...EWR (sorry Catman) I'm a PremEx with UA.