I am reasonably sure (~80%) that no one has come up with a way of finding if a unit is prone, or what stance he is in at all. I can remember a lot of discussion about it, and different methods being tried, from model section-specific drop particles, to focussing a camera on a unit and checking its height, to getting the height of fired ammo using event handlers. None proved to be reliable. If someone has found a way, it will have been recently.
GB is away for a month or two, although I'm not sure, he may still be checking his messages/emails.