Short of an XRF analysis (which provides a print out), there is no definitive way to determine purity.  I also doubt that most pawn shops carry this type of equipment due to its high costs.  It definitely sounds like a case of buyers remorse and is also further compounded by the recent drop in silver price.  Not going to name drop and distract from your thread, but I am a member of a Silver web forum and a few of the members there sell their own homemade ingots (.999) for a premium over spot.  Sometimes the premium is as much as $6 and oz over spot (more then ASE and Maples).  Don't let one bad seed get you down.  Crafting your own ingots is an art form, and it looks like you have been undervaluing your own efforts.  Once you start making custom molds you will be able to increase your premiums.

side note:  Do you use graphite or steel molds?  I am in the process of designing a mold in CAD/CAM and am going to be getting it milled out at a local machine shop and am trying to decide on a base metal.