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?
Agreed. Fudging should always be on the table, because sometimes an encounter will play out in an incredibly unsatisfying way, such as the DM accidentally making combat way too hard. In that case, you could easily fudge a roll and say “yup half the minions failed their saving throws and are killed outright” to get them off the table.