If you totally empty the tank, undo the 2 3/32 grub screws & remove the silver top from the reg (it unscrews, maybe a little tight, but not loctited) remove the spring-stack & count the spacers. A HP system has 12 spacers, a LP reg only has 10.
A bit of a ball ache, but the only sure way to tell. If you need a LP stack, ring Fat Bob's. We keep them in stock for £15.