Any time I go to a bad con game, I have a code phrase I use. If I go with a group of friends to play together, this is specifically helpful to gauge if they're having fun.
"Did Jeremiah have a key to the room?"
If they're willing to give it a bit more time, they will say "I don't think so - but we should be back before he is."
But if it's clear that I'm done, I'll say, "Jeremiah has locked himself out of the room." And if they want to leave they'll say, "I'll have to give you a ride."
Last GenCon, there was a game so bad that I got up, reading my phone, "Oh sh*t! Jeremiah was hit by a car. I'm going to have to go to the hospital."
And of course, our friend Jeremiah actually had locked himself out of the room once. And yes, we still use this code even if he's sitting at the table with us.