How do I know it is Gluttony?

How do I know if I am committing Gluttony? Like if I ate 4 brownies for dessert have I committed gluttony? Is it just overeating? If it is, then I guess I’ve done it my whole life. Would that mean I’m not saved because of the sin? Like I don’t view food more important than God, but I do probably eat more than I should. If I go back for seconds when I’m kind of hungry is that Gluttony?