In the Season 4 episode "Something Blue". After Willow does her spell where Buffy and Spike are engaged the two of them go to Giles' apartment. Spike is sitting in an arm chair next to a glass cabinet with Buffy on his lap. If you watch closely you can see his reflection in the cabinet when he moves. You also see it when he stands up and begins to move away from the chair. As a vampire he shouldn't have a reflection.