Most (if not all) money order providers have a phone number on the MO that you can use to check the balance.

One thing I would recommend for the future is to not endorse the back of the MO until right before you deposit it. In addition to being a clear indicator of whether or not it was deposited, an endorsed MO is about as secure as a blank one. Legally, once endorsed it can be cashed by any one at any time.