Hawk552: Touch might work with func_friction thing but this is more about world brushes with slope faces (> 45 degrees I think). You can't stand or walk on them. As soon as you touch one, entity flag will change to 8 (which is exactly the same value as during falling).
Wilson: Whatever player is pressing and his speed is will not help in distinguishing falling state from sliding. But trace_hull idea sounds interesting. I'll be right back.
Thank you both.
UPDATE:
Trace_hull with with large_hull almost did the job. But then I've noticed that the states where player is in the air + ducked + few units above ground and the state when player is touching a brush (possibly slope) + is above ground are identical in the engine. So another problem - and I don't see a way through trace_hull anymore. This problem is more complicated than I expected.
I possibly have a solution but that includes a bit of vector algebra ... and lot of optimalisations.
Garh >_<