How 'bout asking them?
If you really, really want to get to the truth, and don't care what the potential liar thinks about you, you could try verifying the truth content of the potential lie yourself. Search the basement for traces of blood. Watch the security tapes. interrogate witnesses. Watch a few episodes of CSI. Note, that a blatant display of mistrust like this might offend the alledged liar.
Basically it boils down to how much you trust the someone.