Naturally, it would be quite a task trying to prove your innocence when you have already made the action of trying to purchase something with a Fake Dollar Bill. People NEED to learn and understand that the issue about Counterfeit Currency is not a joke nor should it be taken for granted. And just because you were a victim of Counterfeit Currency does not mean you wouldn't be prosecuted by the law. According to federal law sentences, each counterfeit bill is punishable by up to 15 years in prison. There are a number of methods for detecting Counterfeits.
Some methods involve sophisticated high-tech machinery to confirm dollar authenticity.
You learn them from books and from people who are experts in Currency quality. Even then, you still have to make an effort in using those skills to detect Fake money. Counterfeit bills are typically smooth, much like a photograph which no raised ink can be felt. Counterfeit cash looses their color easily. A small device called the Counterfeit Detector Pen can solve all those problems I've just mentioned.