To sum up here for those who don't want to read back. Your best chances for powder are (in no particular order):
-Skiing at snowy AND remote/unknown resorts
-Skiing lots and lots of days
-Combining as many of these variables as you can
BUT. Our discussion here revolves around point three, and determining how to assess which resorts best fit that bill. All else is really grist for a different mill.
One slight disagreement I have about point three is that the resort doesn't necessarily need to be unkown. Sometimes, just being known for the "wrong" reasons can be a plus.
For example, when I went to Arosa a couple of years ago, I found I had very little competition for powder, even on some easily accessible or interesting lines. The resort is known as a family resort, so even though it's not unknown, there aren't hoardes of powderhounds out tracking out every possible line within 30 minutes of a dump.