With my more experienced dog Dakota, I can usually tell hen or rooster, but that is based on the differences between how hens and roosters react.
With the roosters, they typically run - Dakota will go on a staunch point, soften up, I tell him "free" or give him a hand signal to go ... then this process repeats until we get into a corner or edge and the rooster flushes, or has run a half mile away and nothing flushes! :laugh:
With the hens, they typically hold more often than not. When he slams on point and doesn't budge my first thought is "Hen!"
I am pretty certain that they can smell the difference, and as dogs get more experienced, I think that they would be more inclined to point roosters, since they are the ones that get shot and that they get to retrieve!