The size and redness of the combs is the biggest tell right now. It's often hard to tell by feathers until they are older. I would guess cockerel as well. Having said that, mixes can be more difficult since you don't know which breed they will take more from in any attribute. And I have been fooled before. Sometimes you get a pullet that just has a big comb. And birds that tend towards peacombs or smaller combs can fool you the other way. Not always an exact science. The more familiar you are with a particular breed, the easier it gets with those. Mixes don't always follow any rules. I had a batch of mixed supposedly pullets (barred rock/andalusian) last year, three I was sure were cockerels due to early comb development and faint barring in early feathers. They all ended up pullets. You may not know for sure until they are older.
