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Without a receipt is a bit difficult to track as the system will want this as well as the money order serial number (if you have this then you should be able to track on the USPS website https://mois.usps.com/pls/pmoisnp/mois_external_pkg.main). If the receipt is lost as you have stated, money orders can sometimes still be identified using your checking account, credit card, or debit card information. I would then suggest that you look up the information from the way you made payment and then speak to the postmaster at the office where your purchased the money order. Providing this information may allow the tracking of the money order.
The post office in order to locate the money order if you are concerned that it is lost or has not been cashed will require you to fill out PS Form 6401. Again you will need the receipt in order to fill out the form but speaking to the postmaster as well as any payment information you can provide may help in tracking down the money order.