The opinion of the archaeologists that the mammoth has been killed at Domebo is based on two things:
Weapons; on the site where the mammoth was found the archaeologists also found 4 projectile points, which had the purpose of killing big game animals, and they had markings that suggested that they were used.
Bone placement; the placement of the bones of the mammoth suggested that they were placed in a manner which was used when the flesh of the animals was sliced from their bones.