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?
As a player, I make a point of telling my GMs that I dislike fudged rolls and I’d rather roll a new character than claim a false victory.
As a GM, I will always at session 0 tell my players that I don’t fudge rolls and often prefer to make my GM rolls out in the open whenever there’s a chance they could kill someone or end an encounter. My attitude is that when the players can see my rolls and I tell them in advance “if this is higher than X it’ll hit so-and-so”, we’re all on the same side as we watch the roll play out.
IMO it’s not the job of the GM to tilt the game system itself towards the players, but rather to balance encounters and challenges to be beatable, and then see what happens right alongside the players.
To answer your question, tell him if it affects your experience of the game. Don’t let it ruin your fun in silence, no GM wants players to do that.