And he's a ninja! In Naruto, they do 15 impossible things before breakfast! This sort of line would make sense if Shikamaru didn't personally have the ability to move and kill with his own shadow.

Oh yes, and this.

Shikamaru's initial reaction should probably be something along these lines:

Obviously a jutsu. There were three basic types: taijutsu, genjutsu, and ninjutsu. Taijutsu was obviously out for the same reason it had to be jutsu in the first place. That left genjutsu or ninjutsu.

Consider genjutsu: illusion. Hidan might really be dead, but then he couldn't maintain the illusion; perhaps the illusion was Kakuzu's. Alternatively, the wound could be illusory, or the body itself. Hidan might be uninjured, or somewhere else.

If it was ninjutsu... they were basically screwed. Any technique that could actually prevent death would have to be kinjutsu, a forbidden technique. He'd have to run. Actually, he should probably kill Asuma himself, just in case the kinjutsu prevented his soul from reaching the pure world...

Shikamaru could feel himself flinching away from the thought. No, he told himself. If that was really what was at stake, he had to know. He would not sacrifice Asuma's soul because he was too cowardly to admit that he could lose.

Kinjutsu or genjutsu. Everything depended on this. How could he find out?