At about 2 weeks you can make guesses but they are just guesses...for me atleast until they get 3-4weeks old they can go either way. By 3-4 weeks old you should see pullets have an almost flat comb still that should be yellow or maybe light pink/orangish and the roos will have a larger dark pink/red comb growing in.
Thank you! Right now they're maybe 2.5 weeks old, and their combs are still yellow and barely there, really just pointy enough to know that they have single combs. I guess they should start turning pinkish/red within a week or so if they're little roos.