Personally, I would closely examine their legs, using your hands to look at the breast development and across the back to judge the width and lenght. Feathers can hide a lot. When you have cross bred roosters, you really don't have a standard, however, they still need to be structurally sound. Look for anythinng that is crooked, or out of alignment, checking the beaks for any oddity.
Also check how they walk from the back and from the front.
As a long time cattle owner, I go with this thought, crossbred cows give vigor to their offspring, full blood bulls give conformity, good structure and better genetics. It is worthwhile to spend more on a bull, because his genetics will go to the herd in several cows.
I think that the theory is sound in roosters too. I have had both crossbred rooster, a darling EE, a favorite. And full blood roosters two that were not worht much. Now I have a new fellow that is a full blood. I am headed to a closed flock...I think. What I noticed is over the years, my flock looked more and more less quality. I still got eggs, but the meat was less and less.
Roosters are relatively cheap, it should be rather easy to get a good full blood. In cattle we have done careful genetics. In chickens I just kind of let it happen, however, now I want not so much a pure blood flock, but a higher quality flock... 9 eggs should hatch today or tomorrow, it will be interesting to see how these grow.
A facsinating hoby.
Mrs K