Although I generally only wear one sporran, I have amassed over 20 different types. The closest to sealskin would be bovine. If dyed silver or speckled, bovine could be mistaken for sealskin, but not close-up.

Firstly the hairs are closer together - the seal is an aquatic animal and the close hair is part of the thermal insulation. Next is the colour of the hairs. They should be sprouting from hide that is roughly the same colour. If the hairs are one colour and the hide is uniformly another then the speckles have been sprayed on. Lastly is the feel of the hide. A cow is a slow moving, grazing, land animal, whereas a seal is a supple, fast swimming, fish eating animal. I always get the impression that the bovine is stiffer.

Regards

Chas