There's a gene that makes chickens grow their feathers slower than normal. You can't tell them apart after they finish growing up. Either gender can grow their feathers at either speed.
If you have both feathering speeds in the same breed, you can do the right cross to make feather-sexable chicks. It can happen by deliberate planning, or just by chance.
Most Polish chicks are not the right cross to produce feather-sexable chicks. Whether yours are, I don't know.
Did you raise the parents yourself, from baby chicks? If the father grew his feathers at a normal rate, but the mother took forever to grow hers, then you have the right combination of genes for feather sexable chicks. The difference in feathering speeds is most obvious when the chicks are several weeks old: one set is almost fully feathered, while the other looks mostly naked. But the difference can be seen when they are just hatched, which is where all those guides on the internet come from.