Inspect all bills, especially those of larger denominations for appropriate portraits. Compare the suspected bill to genuine bills of the same denomination. Look for differences, not similarities. Counterfeits will be less detailed, have a flat appearance, and appear washed out.
Authentic bills are always printed on safety paper with fine red and blue hair-like fibers embedded in them. Do not be fooled by colored lines printed on paper.
Help is available if you have any questions regarding U.S. currency. You can contact the local U.S. Secret Service office for assistance.
Common types of counterfeit bills:
Low denomination bills altered to appear high (photocopied corners of larger bills glued to small bills).