Ive been in the Poultry project for 3 years, waterfowl of 6, and the way I was taught is by looking at two things
1. Temperament
2. Best to APA Standard
When showing purebred poultry both of these aspects are very important. Not only will this bird be judged to APA standard (well usually

) but the child will be show the bird on the table, which means he/she will have to pick up the bird and show the parts of the animal + they will also want to show off the breed to the best of there abilities ;show well conditioned bird, to the standard, as well as posing to the standard etc, etc.
Now the hens don't really need to have all those points, mainly just to the best of the standard.
Now lets say you have three roosters, one is gorgeous, a picture perfect animal, but he will take your hand off, well he wouldn't make for an animal for the show table.
Rooster number 2 is friendly, stands still, and is tolerant of the other animals, but he is missing feathers, lost a toe nail, legs are the wrong color etc, etc. Well he may not be the best for the conformation aspect of showing.
Roo #3 is tolerable, has a nice body, close to the breed standard, well he would probably the bird you want to keep, because he is the better of both worlds of showing.
But that was how I was taught, and to each his own!