I do a head count, personally. I don't think there's been one true way everyone has been doing it but if you have a method, stick with it for consistency's sake. Also, I tend to look for the first few instances of loading screens before an install and afterwards before playing the game proper. It saves me from having to figure a good place to start counts all of the time.
So, 27 more titles until we reach the mystical 500, eh? Sounds like a good challenge to me. Here's a few from the Games on Demand service:
Diario: Rebirth Moon Legend - 2.0GB - This is a JP game.
Far East Of Eden: Ziria - 5.9GB - This is a JP game.
Just got Infernal: Hell's Vengeance through Gamefly today. I'll report back later on those findings.