In this particular incident, the "customer" was a bald Hispanic man in his early 30s, about 5 foot 8 inches tall, about 188 pounds. He attempted to pay with the bill (pictured) that passed three of the usual tests (watermark, pen test, vertical strip). He quickly left the shop when questioned by the merchant.
Security features on $100 bills include a 3D security ribbon, color-shifting ink, portrait watermark, security thread, raised printing, and microprinting. Visit www.uscurrency.gov/ to educate yourself and your employees on how to authenticate money and avoid being taken advantage of.
In addition to familiarizing yourself with the security features on authentic money, trust your gut – if something feels weird, ask the customer to provide ID or another bill/payment method.