http://www.bbb.org/alerts/article.asp?ID=583

Postal money orders have many security features to help verify their legitimacy, according to the U.S. Postal Inspection Service. To check that a money order is authentic, hold it up to the light and look for Ben Franklin images repeated on the left side (top to bottom) and a dark security thread running (top to bottom) to the right of the Franklin watermarks, with the tiny letters "USPS" facing backward and forward. If either of these security features is not present, the postal money order is a fake. Also, be aware that denominations appear in two locations. If denomination amounts are discolored, that indicates they have been erased. U.S. Postal money orders are printed on crisp textured paper stock with a maximum value of $1,000; the maximum value for international postal money orders is $700.