At first glance, many of the counterfeit postal money orders now in circulation are hard to spot, but two security features are hard to fake.
Security Thread: A microfiber security strip will reveal the letters USPS in alternating directions when held up to the light. Fake versions may only show a printed bar.
Watermark: When held up to the light, a genuine postal money order will display a watermark of Benjamin Franklin. Fake postal money orders often have no watermark or a crude outline.
(Source: US Postal Inspection Service and lookstoogoodtobetrue.com)