The DM suddenly taking interest in your character is usually a bad sign. "Hey, what's Brisbane Breadcleaver's Fort defense again? He's not immune to poison or, say, demonic soul plague, right?"
Trying not to chuckle too loudly behind the DM screen is also a good tipoff.
Anytime the DM either activates slow-motion on your character or suddenly zooms forward.
"Ok, show me exactly how you touch the Orc statue, using these dolls."
vs
"So you decide to talk with the Succubus, er princess? While you are around the corner making out..."
On a related note, accidental verbal slips from the DM are often the best way to unravel a difficult mystery.
"Your insight check is 15? You find nothing suspicious about the cannibal...Father John. You feel that good priest Father John is a wonderful person."