Reading another post it seems like a lot of folks have gotten burned on fake money orders and didn't know about this automated verification system. It works great, takes just a minute or two, and is probably the best way to double check against a fake mo.
The number is 1-866-459-7822, its all automated. Just type in a few key numbers off the m.o. and it verifies if it is a good one or not. Below is the USPS website that has some good info on their mo's and mentions the number.https://www.usps.com/shop/money-orders.htm
Hope this helps somebody.