I'm new to all of this too and I've been amazed at how fun they are as well.
What I do to find information is search the forum for the topic I want to know about.. for you do a search for RIR. Then, find out who in the forum seems to know what they are talking about. Then you can send a message directly to that person and hopefully they will respond.
Sometimes if I just post a question, nobody notices it and it goes unanswered.
Per my knowledge which is limited, the comb is related to sexual maturity. Based on that, the comb should be in fully at about the same time a hen would begin laying or shortly before this. I think that is at about 20 week for a RIR.
If it comes in quicker than the rest of the group you have, it may be a rooster.