So everything is going well. It's the rare occasion that the other couple isn't composed of one cool party and one dud. Wine is flowing, people are laughing, everyone is having fun. Then it happens: once that check comes it becomes get out of Dodge time. The rush that just came over tells me that the person is splitting, not because they are having a bad night out and want to part ways, but because there is a rumble in their stomach and they have to unleash it on the closest toilet. The pressing plop (usually caused by a rich desert) reveals itself when one half of the couple wants to keep the night rolling and go out for a drink and the other is adamant about leaving with a frightened look on their face.
Rule of thumb: if you aren't going out for drinks after dinner: you're laying a log.