As the economy declines, the percentage of fake currency in circulation will increase, just as it has in economies that have experienced collapse (like Argentina). Although banks have machines that scan incoming currency for counterfeit notes, that does not mean that you can trust a bank to give you authentic notes. Every time you receive currency, take a moment to scan it for signs that it is fake. The United States Secret Service has a good page that describes how to detect fake currency.

The U.S. Treasury provides guidelines on how to respond when you think you have been given counterfeit currency:

• Do not put yourself in danger.
• Do not return the bill to the passer.
• Delay the passer with some excuse, if possible.
• Observe the passer's description - and their companions' descriptions - and write down their vehicle license plate numbers if you can.
• Contact your local police department OR call your local U.S. Secret Service Office. EMAIL US at this address, or contact the U.S. Secret Service through your local Field Office found in the U.S. Secret Service Locator.
• Write your initials and date in the white border area of the suspected counterfeit note.
• DO NOT handle the counterfeit note. Place it inside a protective cover, a plastic bag, or envelope to protect it until you place it in the hands of an IDENTIFIED Secret Service Agent.
• Surrender the note or coin ONLY to a properly identified police officer or a Secret Service Special Agent, or mail it to your nearest U.S. Secret Service field office.

We're not experts on detecting counterfeit currency, however, people who do it regularly (as FerFal describes in his blog Surviving in Argentina) notes that there are in general two ways to detect counterfeit notes. The first is to use a ultra violet light similar to that described here. The second way is to examine each security feature the note has as described on the Secret Service page above or at FerFal's post here. FerFal does not use nor does he recommend as the sole test a counterfeit detection pen.