Dean will have a hard time guessing which event(s) in particular they're referring to because there are... an unfortunate excess of them, so he's going to do his best to generalize the most memorable possibilities with probably a high 2 on the convincing scale. He'll say his side of the story is that things started as a verbal altercation after everybody had a little too much to drink, the folks he got in an argument with didn't like the way it ended, so they waited until Dean was alone and not in Mag's to turn things from verbal to physical. They bit off more than they could chew, a fight ensued, and as far as he knows everybody walked (or limped) away.

The exception to this that could make it potentially a lie are a handful of bandits way out in the desert who instigated a fight and wound up dying, but that was far enough away from the city proper he doesn't think the guard would know about it, or that he was involved.