I've heard that Polish cockerels have smaller crests than pullets. I never go by this, though.
You're best bet is to wait. You can try to crest sex the babies, but that often give me ALOT of cunfusion I don't need. I tend to wait like everyone else until they are totally feathered out and their top-hats are nice and flowy. At this stage, everyone's head looks around the same. Feathers are starting to 'sprout up' and make them look like males with their dread-locked feathers. Totally not accurate. Be sure to look for the start of wattles/combs. Also look at behavior, like every other chick. People tend to think crest sexing makes the deal a whole lot easier but it is a very misleading method.
ETA: These pics are older than 4 weeks but some examples may bring a clearer mental image.
Okay, not a Polish, but this is my Sultan cockerel at around three months. (Told you I didn't have many pics, didn't I?)
When they are completely feathered out, you my notice the start of drooping tails, pointed saddle/hackle feathers, and the crest is a dead give-a-way.
This is Tosca, the Sultan's daughter, at around 6 weeks. She is Polish X Sultan, but retains the Sultan's smaller crest. Watch in the next few weeks for rounder, dulled but poofy feathers on the body, but mostly the crest. Remember, wait until the top-hats are nice and full, grown out. It takes time before you know for sure.
I hope this was a bit more helpful.
