Hmm, the best way to tell would probably be to gradually warm up the water. You don't want to put him in to 75 degree water immediately because he can go in to shock. You will want to slowly warm the water over the course of 2-3 days I think until you get to 75 degrees. He should come to shortly after if he's still alive.
It also seems that, through my brief research, turtles have strong heartbeats. You could try listening for a heartbeat, though it is likely to be very slow, according to this website it could be once every 15 minutes! (
http://www.stoller-eser.com/nie/Weeks/AAS4-14LR.pdf)
Unfortunately if he has started brumating, it will be nearly impossible to tell if he is dead or alive until he comes out of it. Give it a few days or listen for the heartbeat.