Which one are you trying to figure out if it is a roo or a hen? They both LOOK like hens to me but looks can be deceiving. But if one of them have extremely large combs it just might be a rooster. But a rooster has a totally different noise then a hen. It will start making 'baby' crows and it will be more aggressive toward the other chickens.
Well I hope whatever the outcome is you will be happy. But I had the same kinds as those in the picture, although I don't remember what kinds they were, but they both looked like that and they were both hens. However I have had chickens for only 4 years so I would let those who have had chickens longer advise you!