Two kinds of belt tensioners?

[George Davis]

BMW dealership said the 1994 525i uses two kinds of accessory belt tensioners (main belt, not a/c), mechanical and hydraulic. Aside from getting in there with a mirror (which I don't have with me), is there an easy way to tell which one I've got? FWIW, mine has a hex-head bolt in the center of the pulley, not an Allen bolt.

Thanks for any help!