1. General look of the coin is to be considered. A lot of fakes have pitted surfaces or other marks that don't look correct.
2. Reeded edge; many counterfeits didn't get this correct and it's obvious when you see it.
3. Weight; Of course weight must be considered and usually the fakes are underweight.
4. Sound of the 90% coins when they are dropped, compared to a new coin
5. Size and thickness; there can be wear and tear, but it should not be too bad.

Magnetic Properties of 90% silver coins when interacting with a strong magnet

Weight of 90% silver coins based on the minted weight of coins as well as the density of the materials

Sound Properties of 90% silver coins when dropped onto a hard surface