Underweight: the head will be wider than the fishes body
Overweight: the fishes body will have a disproportionally large pot belly (please note that certain kind of fancy goldfish such as ranchu, pearl scales and lionhead naturally have a pot belly)
If you mean like what breed of goldfish you have then that should be fairly easy. I determined what kind I had by looking at his markings and going on wikipedia. If you go to Wikipedia.com and type...
Sexual dimorphism in goldfish can be hard to detect. Typically one cannot determine a goldfish's sex unless it is sexually mature and conditioned to breed. During the mating season, females will grow...
You can tell if your Body Mass Index is above 22.5 and your body is mainly fat not muscle. If you are female, no more than 18-20% of your weight should be fat. If its more you are over fat if it is...