1 Jamie: An interview doesn’t sound very vampiric. We need to run some tests and find out if you’re really a vampire or not.
2 Jamie: We can see if you cast a reflection in a mirror. Film it for a Mythbusting episode.
3 Adam: That doesn’t sound like very interesting TV.
4 Jamie: Or we could stake you through the heart.
4 Adam: That’s the sort of daredevil Mythbusting we like!

I mean, if you want to test if someone is really a vampire or not, you don't want to use some inconclusive method like checking their reflection in a mirror. There could be any number of reasons why a person casts no reflection. They could be a ghost, for example, which is a totally different form of undead to a vampire.