I know. The method I described is for detecting whether or not they just jumped. If they're on the ground, they didn't just jump. Once they leave the ground, you can check ySpeed to see if they actually jumped or just walked off a cliff, etc. Obviously there are further details (players aren't 1 pixel thick etc; you wouldn't actually want to use jjMaskedPixel, because of platforms and such) but surely you can figure those out.

I'm also confused as to what you intend to do with this information in the first place...