[AFFA]

I got a pretty old silver coin (claimed high Ag purity) from someone. How could I determine its purity briefly (high or low) without destroying the coin???

Suppose that I cannot use any complicated instruments.  

[Borek]

For classical methods you will need to dissolve sample in acid, which is not an option. So you are left with instrumental techniques, which are not an option either. For me that's a no-go situation.

[Dude]

The best non-detructive technique to get the metal composition is X-ray fluorescence spectroscopy.  Essentially every other technique available would require dissolution of the coin (destruction).  Unfortunately, XRF instruments fall into your expensive instrument category and I'm probably not being much help.  If you are really interested, check your local university and you might find an academically interested professor that has one of those instruments run it for free.  

[Alkrav]

mm.... how does the surface of the coin look like?