How do I know if I have body dysmorphic disorder or am I just fat?

Weight tables. You can compare yourself to insurance weight tables. http://www.halls.md/ideal-weight/met.htm do note that it has more recently been shown that a weight that is 10% over the insurance weight tables is more healthy as you have some reserves in case of serious illness that interrupts nutrition. If in doubt, see your physician who can do a better assessment.