QUICK! How do you tell if an opossum is playing dead or actually IS dead!!???!!!!!?

so it's the night-time and i let my dog out to go to the bathroom, and he wouldn't come back. So i went outside to check what was up and i found him in the compost with an opossum lying in a dead position next to him. my dog is a terrier, so he goes for anything that moves. At first i was freaked that a beautiful opossum had been killed for no reason, but then i remembered that opossums play dead in order to escape from their predators. soooo my question is, is this opossum playing dead or is it actually dead? the opossum is lying on its side with it's eyes open and it's still breathing, but i dont know if it's dying or not. if it's injured and dying then i would like to help it recover if i can in any way, but if it's just playing dead then i'll just leave it alone...! thanks a lot!