I'm still fuzzy on what constitutes a "credible" vs. non-credible death threat. Specific details?
If it is phrased as a joke, then it is not credible. If it is an impossible scenario, then it is not credible. "If you don't shut up I'm going to send a bunch of Cardassians to your planet!" Or if they admit they don't know where you are, then it is not credible. "If I knew where you lived, you'd be toast!"
You can get the whole thing from the semantics of the words "credible" and "non-credible," by checking if it is non-credible. If it has something as mentioned above that makes it "non-credible," then it is not credible. If it doesn't contain only of those things, and is just a threat, without anything to discredit it, and it contains claims of having access to you, such as knowing your address, then it is clearly credible.
Note that "credible" doesn't say, "actually planned and they're at the door" or anything like that. If it is not non-credible, then it is credible; if they make it sound like they might really do it, then that is clearly and indisputably a "credible" threat. But if you can't tell, then it is credible as a threat.
Dictionary gives "able to be believed; convincing." And: "capable of persuading people that something will happen or be successful."
Synonyms are: believable, plausible, tenable, able to hold water, conceivable, likely, probable, possible, feasible, reasonable, with a ring of truth, persuasive
A threat such as, "I know where you live in Sometown, Somestate and I've been tracking your location and I'm gonna [felony] and [felony] you!" Clearly plausible as a threat; you'd have to know independently if they are or are not stalking you in order to determine it. But the claim itself is that they are stalking you, so the stalking part is plausible, and if they are stalking you, then the rest is even likely.
"You're so lame, if I met you I'd totally [felony] you." Not very plausible as an actual threat.