This is going to sound weird/maybe stupid but i've been laying low and not betting and keeping track of this.
If there is a match on a big TV station like ESPN/ESPN2/CBS I immediately check to see how long the coverage is supposed to last after the first set is done. If it is listed as another 1.5 hours or 2 then it has been going 3 sets everytime.
I've noticed this in both Caro/Serena matches and a lot of the Venus matches a couple weeks ago. I also noticed that coverag ewas only listed as 1.5 hours in the Isner/Anderson match that ended in straights.
Call me stupid but I just placed a wager on Rosol to win the 2nd set after I noticed the guide said coverage would last until 3 and he pulled it out, even avoiding 3 break points.