Another option I see¹ is to inspect the source-code.

Every internal error-msg comes with parsable file and line infos. You could try to match for "die" there.

Saying this I have to tell you that file and line infos can be manipulated.

OTOH combining all shown approaches should make it rather difficult to fool your monitoring solution.

¹) beside hacking CORE::die  with XS (which of course can be sabotaged by modules hacking CORE::die  ...)