Just start unscrewing the co-ax slowly out of the socket. If you get snow/doubles before it breaks up completely, it's analog, if the picture freezes with blocks on the screen, you have digital.
ReplayTV 3000 (RIP)
Hughes HDVR2 DTivo (adopted by friend)
Tivo S2 Dual Tuner (Craiglisted)
Tivo S3 750GB (decommissioned)
Tivo HD 750GB (adopted by parents)
Tivo Premiere XL