Here are some good resources for identifying the most common forms of counterfeit currency.
“WASHED” bills are $1.00 or $5.00 bills bleached and reprinted with a higher denomination. These bills can beat the “counterfeit detection” pens because they are on genuine paper. You can find an article on these bills and how to detect them here: Detecting Counterfeit Money: Washed Note Counterfeits (fraudfighter.com)
CHINESE TRAINING BILLS are notes created to train Chinese bankers how to identify the different denominations of American bills. These have bold Chinese characters that roughly translate to “teaching money, do not spend.” Here’s more information about them: Counterfeit bills marked with Chinese letters being circulated – WFMJ.com
ALL OTHER COUNTERFEITS will tend to have easy to spot characteristics. This link offers 8 Ways to Spot Counterfeit Money (losspreventionmedia.com).
Always be wary of people trying to spend a large bill for a small purchase and take the time to examine bills you receive. If you believe you have been offered counterfeit bills do not accept them and contact law enforcement.