He will certainly be able to see differences! You're spot-on about the human+ makeup of the blood and bones— it's a strange mixture, some of it matching the person whose double it is and some it just... different. Wrong, somehow. They rot in the same way but sped up, and the death energy that lingers around them spikes when they die but stays high afterwards. It practically overflows from the double's bones, and there's something distinctly Off about it— an energy that John, specifically, will recognize after being abducted. It feels the same.