why don't you use a blacklist, similar to what kbot does for looting corpses, so that it doesn't loot the same corpse twice. I did a similar thing to vgshaman so that I don't attempt to harvest corpses more than once. Your other option is that if you attempt to do whatever necro ability it is on an invalid corpse, does it give some type of message back, i.e. "you can't use this ability on this corpse", that can be captured from the onincomingtext event.
If all your interested in is a yes/no flag on the pawn object, it's probably not there. Ama would probably need access to a necro with this ability to put that functionality into isxvg and he probably doesn't.