Touch the twenty dollar bill and get a feel for the paper. Some of the most obvious counterfeited money's paper has a very different feel than the real twenty dollar bill. The paper is usually thinner and has a different shade of color than real money. This requires some experience because it is almost impossible to fake the texture of a real twenty dollar bill. If you cannot tell any difference, compare it to the twenty dollar bill that you know it's real. There is more than one texture on the twenty dollar bill; his cloth has a texture of lines.
Use a counterfeit detection pen. It is very intuitive to use. Just mark on the twenty dollar bill and look at the color afterward. If it's yellow, then it's really money. If it turned black or brown, it is a counterfeit bill.
Look for a security thread. Hold the twenty dollar bill to the light and look for a security thread, it should read "USA twenty". Another thing is the thread glows in green under UV light.
Check for watermark. Again hold the twenty dollar bill to the light and look for the watermark. The watermark should be seen from both sides because it's embedded in the bill. The watermark should be the same president on the bill, Andrew Jackson.
Familiar yourself with the current twenty dollars in circulation and avoid being a victim of counterfeiting. If you bring a counterfeited twenty dollar bill to a commercial bank for deposit, you will end up losing the money.