How do you know if you have body dysmorphic disorder or really is just ugly?

Preoccupation . I think it has more to do with how a person can become preoccupied or obsessed with their appearance. For some, the feelings of being ugly is all encompassing and a person may waste hours a day worrying about their how they look or some imperfection.
Opinion of Others. One answer to this is to see what others say about the person. If a person is convinced that something about their body is "wrong" or doesn't look right, but the people around him/her disagree, the the possibility of body dysmorphic disorder should be considered.