Quail, right?
I almost answered in a chicken-specific way, and then saw that this is in the quail forum. I hadn't quite realized that hens, roos, and crowing were all appropriate terms when dealing with quail too!
For any species, I would start by removing any that have problems, that cause trouble, or that you just don't like. So check each one over for physical issues (crooked beak, missing toe, walks strangely, always smaller than the rest, etc.) And notice whether any attack you, or are mean to the females, or always get out of the pen and open the door. Think about whether any needed extra help as babies: assisted hatching, poopy butts, slow to find the food, and so forth.
Think about what temperament traits matter to you. For example, if you pick one up and it sits calmly in your hand, while another struggles wildly. If you reach into the pen and one runs frantically away while another is fairly easy to grab. If one panics at every new thing, while another accepts life calmly. (Personally, I like birds that are relatively calm, and easy for me to handle. But you may not care, or may have different preferences.)
I usually find it easier to decide who goes, instead of who stays. (Same decisions, just a different way to think of it.)
And the more you remove, the easier it is to notice details about the remaining ones. So then it can be easier to sort them out too.
I think that would also impact you’re decision. 
