There are a few simple steps that businesses can take to verify whether the currency they are presented with is genuine.

1. Feel the texture of the bill. Paper money is printed on cotton rag and has a distinctive feel. (This can also be verified with a counterfeit detection pen. The ink from the pen shows as a light yellow if the paper is valid or as a dark color if the paper is invalid)
2. Compare the bill with another of the same denomination and series. Look for differences rather than similarities (1 difference could mean counterfeit)
3. Use a UV lamp. Each bill has a watermark face and a colored strip that glows under UV light. (100:Pink, 50:Yellow, 20:Green, 10:Orange, 5:Blue) Often counterfeiters will wash a good bill and reprint a higher denomination over the top. Make sure the bill you are verifying has the correct color strip and watermark image (this may also be verified by holding the bill up to the light although the color is more difficult to determine).
4. U.S. currency is printed with a raised magnetic ink. Run your finger nail across the bill. You should be able to feel the distinctive ridge marks. Keep in mind that the magnetism of U.S. currency is very low. This method of detecting counterfeits is generally only useful when using a currency counter or a powerful magnet such as Neodymium.
5. Use a magnifying glass to examine micro printing. Often counterfeits will contain blurry micro printing or none at all.

Also consider the age of the note. The average U.S. note passes through the Federal Reserve once every five months. Every time a note with an outdated series passes through the Federal Reserve it is shredded and replaced with one of the current series.