If you are pulling from a straight run bin, there is a trap waiting for you. Even at that age the cockerels have more personality. They are the ones that will be curious or brave and walk toward you instead of hanging back. They might position themselves between you and the rest, trying to protect the flock. They stand more upright and look very alert. You are going to fall in love with them they are so adorable. You might want to pull chicks from back in the mix, not the ones that stand out. Cockerels often stand upright, with their heads held high. Pullets are often more level to the floor.
Not all cockerels look or act this way. Most probably don’t. There is no guaranty picking chicks this age but you can improve your odds by avoiding the obvious ones. It also helps to be there fairly soon after they come in. Some people are pretty good at not picking cockerels so if a few of them have been there first you may have mostly cockerels left to choose from.
My local is a Tractor Supply, that’s the one I deal with. From reading on this forum it’s obvious that different Tractor Supplies have different people managing chick days. Some know chicks, some don’t know that much. I feel that mine take care of them properly but aren’t very knowledgeable about breeds and such. They do have a fence built around the bins so a customer can’t just pick up a chick and put it back in the wrong bin. The chicks are not constantly being handled or mixed. Different Tractor Supplies get them from different hatcheries too. There are some national Tractor Supply policies but a lot of local management is left up to local management.
Good luck on picking your chicks. That should make for a pretty flock.