One thing I always look at is its feet! The rooster will have a sharp-pointed spur on the back of each foot just up a couple of inches from the ground and they point inward. (This makes him a tough guy!) They get bigger as the rooster ages. But they don't really show up in young roosters until around the time they start crowing. Good luck!
We have a flock of 12 RIR pullets, and one RIR rooster. The differences were fairly easy to see early on ... development of longer tail & neck feathers, and the larger comb. But, if yours are all pullets, then you don't have anything to compare to. It's likely that they just aren't old enough to start laying. Our pullets *just* started laying (yeahhhh!). We were gifted with our first egg the day after Thanksgiving; the pullets were exactly 19 weeks on that day. I think we now (1 week later) have two hens laying. We had 1 egg Friday 11/28, 1 Sunday, 1 Tuesday, 1 Wednesday, 2 Thursday, and 1 on Friday. So, egg production is underway! Now, if only I can convince them that the nest boxes are the ideal location for egg-laying