You used to be able to pick up a CD and tell instantly what genre it would fit into. With the "progression" the scene has taken over the last few years though, most bands could be a mixture of both. There are still a lot of obvious ones out there though. Both styles use harsh vocals, blast beats, move slow as well as fast, melody (or just plain brutality)... Listen to some of the extremes from either direction, and pretty much follow the "atmosphere".
Death- Severe Torture, Krisiun, Divine Empire, Mortician. . . . .
Black- Abigor, Immortal, Dark Funeral, Limbonic Art. . . . .
I'd go with "Death metal has deep vocals" and "black metal has higher vocals", but even that doesen't really separate most death/black metal anymore these days...