You don't mention how old they are.
They are not sex links so you cannot use color or pattern.
You don't know if the genetics were set up right so you can't use feather sexing. You might get a lot of misinformation on this one.
It takes a lot of training and experience to use vent sexing.
I don't know of any practical way for you to be sure. There are some general trends, such as roosters sometimes have bigger legs and a more upright posture, but these are just general indications and not for-sure methods. With RIR's, you can probably get a pretty good indication around 4 weeks, which is when they are pretty well feathered out, but that is not always for sure. Some breeds and some individual chicknes are harder than others.
Not much help, I know.