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:
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.