If you have a copy of the OG, that's a great way to gauge difficulty level (each section goes in approximate order of difficulty). 500 level questions are in the middle, and 600-700 level questions are towards the end. The math in OG11 is a little easier/different than you'd see on the real GMAT if you were scoring in the 80th percentile or above in quant - not sure about OG12 as I haven't had a chance to do the problems yet. But still, it's good practice and a good way to test your upper-bin ability.

Kaplan 800 does not contain terribly difficult problems. Kaplan is great for students aiming for 500-650.