Its very very very easy to tell the difference between a button buck and doe fawn when they are in bow range. Not to mention, that depending on the time of year, (especially around the rut), if you see a smallish deer all by itself, you can hang your hat on the fact it'll be a button buck run off from its momma while she makes him a little brother or sister.

Button bucks have short faces, round heads and short lower jaws.... and of course... buttons... which are usually fairly easy to see.

My golden rule is never shoot a fawn if its all by itself... and I've never shot a button buck while bow hunting.