If that doesn't work...just logically thinking...you could test to see if they have velocity, make sure they are on ground, and check if they are not pressing any movement buttons
If I remember correctly, you have to hold in buttons to surf though. So you'd have to compare the buttons to which direction and velocity and agh...
Actually re-reading what you're saying...if the entity flags say he is in air when he is jumping, just do a trace-hull of a slightly larger space and see if you hit any ground.