Well the first step is to identify fawns. Obviously they are smaller. They are square in build, without the sleekness of an adult doe. It helps to have them in a group in order to compare then to others. Also, their heads are short and boxy, with large ears.
Next up would be the yearling class. They tend to be fairly small as well, but have longer heads and a more athletic appearance than fawns. They tend to have tight mid-sections and are very light on their feet (if that makes sense).
Beyond that, I tend to guess the age of older does baised on size, length of the head, and dominance in relation to other deer.