How can you determine if a piece of currency is counterfeit?

To determine if a piece of currency is counterfeit, you can check for security features such as watermarks, security threads, and color-shifting inks that are present in genuine bills. Use a UV light to see if the currency has embedded fibers that glow under ultraviolet light. Additionally, feel the texture of the paper; genuine currency has a distinct texture due to its unique composition. Lastly, examine the printing quality; counterfeit notes often have blurry or smudged details compared to authentic ones.