I assume you want pullets. If you knew the specific breeds in the bin, we might be able to give you some hints that might help, but what would help on one breed would be the exact wrong information for another. For example, if the chicks were Barred Rocks or Dominique, you should get the darker of the chicks and the ones with the smaller spot on the head. But if black sex links are in the mix, I've just described a black sex link male.
In general, try to get a healthy active chick. Avoid the ones that are standing by themselves sort of hunched up. Also try to avoid the ones that have poop on their butt.
For pullets, try to avoid the ones that have a tall proud upright posture. Those might be young roosters.
If broilers, Cornish X or whatever they call the meat birds are in the mix, I'd stay away from the yellow ones. You might be passing up a nice white leghorn that way, but I would not take the chance.
It depends on how they have the chicks split up too. If they have a pullet bin, most are probably pullets, but there is still a decent chance that you can get a rooster. The hatcheries only guarantee sex being accurate 90% of the time, and that is only on full sized fowl. They don't even try on bantams.