Well, I would imagine fairly low. It would probably a matter of "Is it still breathing?" If it's just a normal person and you could see chest movement from breathing, or can feel them breathing if you put your mouth close, then it's fairly obvious.
That said, it would most likely be Heal. We've always houseruled it around DC 10 or so. If basic first aid is DC 15, checking a pulse should be a fair bit easier, barring abnormal situations.
For example, back in the day, silly commoners used to drink from lead cups. Sometimes they'd just pass out and go into a coma, quite close to death. No one could figure out whether or not they were dead, so they'd stick 'em on the kitchen table and wait for them to wake up. That's actually where the ritual of holding a wake came from, I believe.
Hopefully your PCs aren't quite so . . . feeble . . . but DC 10 should do in a pinch, I'd say.
Pathfinder Adventure Path Charter Subscriber; Pathfinder Comics Subscriber; Pathfinder Campaign Setting, Card Game, Companion, Maps, Modules, Roleplaying Game Subscriber
In 3.x the DC to determine if someone was still alive with a Heal check was 15. Considering the DC to stabilize a dying character was also 15, and I never saw anyone who thought it through ever bother to check and see if they were dead or not -- just roll to stabilize. Both are standard actions, too.
As an EMT in the real world, it is usually unlawful for me to pronounce someone dead. While there are obvious cases, they must meet A strict set of criteria. It is easier to perform CPR than to decide if someone is dead.