Sometimes I can tell when my current DM fudges a roll to miss an attack or reduce damage. He has a tell in the specific way he pauses and breathes before announcing the roll, then tries to hurry to the next turn, which only seems to happen when someone is in a life-or-death scenario, but “luckily” survives.
Should I let him know he has a tell? Will it be less fun (or more stressful) for him if he knows I know?
Not necessarily true as there are entirely different reasons for doing each. Only a bad GM would fudge a hit that actually penalized you. A good GM might fudge a hit to take the story in an interesting direction. Fudging rolls is just another tool in the box.
I don’t disagree with your line of thinking, but OPs paragraph there says it’s when a character is likely to die.