It's more a question of determining whether it's genuine. Very good replica bills have been printed for about 80 or 85 years, using artificially aged paper and photocopied printing. So many have been made that there's an 80 or 90 percent chance that any given Confederate bill is a replica. They aren't technically counterfeits because they were sold as novelty items, but over time a lot have made their way from display cases to the open market.
Replicas made in the 1970s and later must carry the word COPY by law, but earlier ones are almost indistinguishable from the real thing. You'd need to take it to a currency expert who has the experience to ID the real thing.