The nature of counterfeits is that whenever someone comes up with a good way of recognizing them, the counterfeiters find a good way to get around it. That's part of the reason for the changes in our printed money.
When ever you see a One Dollar Bill, in the front of the bill there four numbers, Each one next to the Number One at each corner each number the same. On the left-hand side you will see a small circle and one letter inside. If the number at each corner does not count to the letter in the alphabet four 13's and one M, per say, it's fake.