I invested early-on in testing tools (and knowledge). Weight, dimensions, and ping test are quite effective. I also got one of those Sigma Metalytics testers (which still requires the other tests to be conclusive). Another thing is an ultrasonic thickness gauge, which basically tests the same thing as the ping test (celerity, speed of sound in the medium). I've bought from private sellers and online ones, never once got anything that didn't pass.
I picked up a Sigma tester a few months back just for peace of mind.
BTW assuming you don't have the pro version, the classic version has a which displays a number of the raw reading from the probe (hold select while powering up). It's an interesting way of identifying various metals by their resistivity, not just precious metals.
I picked up the Pro mini…I don’t think it has a measurement mode. Still works well, although for some reason they don’t have a setting for .999 silver, just .9999. My silver Krugerrands always read a little off, but other than that it works well.
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