Basicly, no...
You can't
You could maybe make a new addon, that is a really weak bullet (No dammage), and make it at different heights at the player.. And if it hits at z height of 5.. They're standing up.. If at z of 3, hits, they're crouched, if at z = 1, they're prone (In that order.. The crouch one would hit a standing person)
Something like this :
make weakbullet
setpos bullet
set velocity towards player
player add a hit event handler
if hit, remove eventhandler, quit scipt
Etc
Dunno..
- Ben