The question of how to identify a turkey as a hen or a gobbler, male or female might seem like a silly question to a seasoned veteran, but to the beginner, positive identification is a skill that doesn't come just because you decided too take up the sport. A little study and time in the field will have you picking out the gobblers with the best of them.
The first most obvious difference between hens and gobblers is the beards that the males/gobblers have. This is often the easiest method to identify gobblers but in some instances hens (~10%) sometimes have beards too! So use this as you first indicator but just keep in mind that it isn't fool proof.
A gobblers body size is often larger and this can help when a gobbler is with hens but judging size, especially in uneven terrain, of solitary birds is difficult at best and could fool even the most veteran hunters. Fortunately we as hunters have a number of different indicators and don't often have to rely on just one indicator.
The gobbler's head adornments are not readily visible until his second year but first year birds (jakes) are not favored or even legal in many states anyway so the head coloration is a good indicator of shootability especially in the spring. In the spring, it is relatively easy to distinguish males from females. Gobblers exhibit bright red wattles and light blue cheek patches. This is the best way to identify a turkey from a distance.
The only sure way to tell a gobbler from a hen is to examine the lower breast feathers. The tips of male turkeys are black whereas in hens they are white or buffy colored. Although differences can be seen at some distance if binoculars or a spotting scope is used, sex identification via breast features might not be useful when gobbler or spring seasons are underway. Once you have a bird down in an either sex fall hunt, the best way to identify its sex is by the breast feathers.