Well, if you have your PS3 set to 720p, then your TV does the scaling, if you have it set to 1080p, the PS3 does the scaling. That's what I've always understood.
If you really wanna check lag, you could try using RockBand 2, or any Rockband game after that(Like RockBand beatles). It has a calibration tool you can use there. Either automatic(with the Rockband guitar and mike), or manual. It's not as accurate as other methods, but I hear it gives you a good idea of how much lag your TV has. Try it with all PS3 resolutions, too.