20 for 20
For the most part, these weren't too hard to tell apart. The Civil War era images are generally much more haphazard in composition, the contrast is higher (whether through reproduction or plate quality), and the environments are generally more complex--oh yeah, the hair is usually wilder.
The reenactment shots have a better technical quality, the poses and compositions are neater, a little "artiness" creeps in, the soldiers a little less ragged, and the scope of the environments is more controlled and self-contained.
There's also something about the facial expressions--maybe a bit of self-consciousness in reenactors.