How do you tell if a Confederate bill is real?

Take it to a local currency dealer; more than likely they will be able to appraise your money and tell you if it is real.

However, be aware that about 80 to 90 percent of the "Confederate" currency being traded outside of core numismatic channels is fake. There were millions of replica bills printed and sold in novelty shops, at Civil War museums and battlefields, and given away in various advertising promotions.