Determining whether or not a money order has been cashed varies depending on where the money order was purchased. USPS and Western Union offer tracking systems, but other locations may require the purchaser to visit the store and fill out a tracking form. Most companies also charge a fee.
USPS offers an online tracking system that allows the purchaser to enter the serial number from his receipt to determine whether or not the money order has been cashed. Western Union provides an online form that one can print, complete and submit via mail or fax. A receipt for the money order is not required, but the fee for tracking the money order varies based on whether or not the receipt is available.