How can you tell if an old 20 dollar bill somebody in the window?

To determine if an old $20 bill is genuine, check for key security features such as the watermark, security thread, and color-shifting ink. Hold the bill up to the light to see the watermark of Andrew Jackson and verify that the security thread is present and glows under UV light. Additionally, inspect the fine print and look for a clear, distinct portrait with sharp details. If the bill feels unusually smooth or has a different texture than current bills, it may be counterfeit.