It should be simple. If your commander says here is a gun, ammo, and you have to wear your helmet and body armor, then you should get combat pay. If you are a pilot and they give you a pistol, ammo, a blood chit, and a much better survival radio than you get in the States, you should get combat pay.
I used to be combat zones were defined geographically - you chop in on orders you get paid while there you chop out it halts.