Bell's best known for his 2001 work, Food for the Dead: On the Trail of New England's Vampires. It's one I highly recommend, by the way.
Anyhoo, here's a snippet from the link:
THE BELIEVER: How do you know when you’ve found a real vampire?
MICHAEL BELL: Well, when you get familiar enough with the vampire tradition, there are certain cues and motifs—little narrative elements that stand out—and make you realize “OK, this is probably something I’m interested in because it’s similar to the vampire tradition,” and then there are other elements that seem to be directly from popular culture that don’t fit, and you can tell pretty quickly one from the other. Basically I look for cases where the people involved were dying from a specific condition—where they exhumed the bodies, what they did to the bodies—cutting out the heart, et cetera. Those are the things I’m looking for.