Sokolov would have had to use the primitive forensic methods of the time. There probably would have been enough to recognize at least some of the victims, remember it was icy at the depths they were burying the bodies. There were also the pieces of clothing and personal items they located nearby, which was pretty conclusive, actually. Find the remains of their clothes and personal effects, which first hand witnesses can identify, then find the right number of mostly intact bodies a short distance away, and well, res ipsa loquitor.