Fawns have much shorter faces than adult deer. Visually, the distance from the tip of a fawn's nose to the center of it's eye is almost equal to the distance from the ear-hole to the center of the eye. On an adult doe, the distance from nose-tip to eye is twice the distance from ear-hole to eye. Other visual clues include: fawns have a much shorter body trunk than adult does, and fawns have shorter necks and smaller ears than adult does.
But the best advice to reduce the chances of taking fawns (and button bucks specifically) is to not shoot lone antlerless deer. A deer by itself provides no size reference. A lone fawn can look much bigger out by itself than it really is. Another piece of advice, don't shoot the first antlerless deer that walks out into an opening, field, or food plot. Fawns are less wary than adult does and are usually the first to enter a feeding area.
Below, the first two pics are first, an adult doe and second, a fawn. Notice how short the fawns face is. The ear-to-eye and eye-to-nose distances are about the same on a fawn. the third and fourth pictures are a doe and fawn looking at you. The ear-to-eye and eye-to-nose distance differences are still quite apparent even when the deer look at you.