We figured that Alceste was the protagonist since the play is titled after him, but that's not the only clue. Molière is not too subtle, and there is no way that such a cranky guy without power, money, or anything could have three love interests without being the main man of our story.
Plus he has an awesome best friend! The two non-horrible people in the play are in love with him! What other signs do you need?
Ok, sure. Protagonists are usually awesome people that you wish you could grow up to be some day, and no one wants to grow up to be Alceste. But let's focus on his good points. He's moral and he stands up for what he believes in. Most of his stupid antics make us sad for him or laugh at him instead of hate him. Since we can kind of empathize with Alceste, we're going to keep him in the protagonist zone.