17, 5 month old chickens. The roos by this time should be larger than the girls and most likly crowing. Check for hackles and sickles. Best thing if you are unsure, is post a few group shots and we can probably pick out the ones that are roos, or would need a closer inspection.
Check their legs for the quickest sex sort. The males will have little bumps on the inside, a bit above the toes, that are the beginnings of their spurs. These start growing at about 3 months, so you'll see definite differences from the females, who just have a spot, no protrusion.
You can check Google for sites that describe sexing the birds by looking in their vents. At 5 months, this will be a lot easier than doing it with small chicks, as the males will be nearly sexually mature if not ready yet. (I've had some precocious cockerels that were into courting the girls by 4 1/2 months.)