Two ways, but neither is expecially satisfactory.
Firstly, when spun on a metal surface and allowed to come to rest, the debased silver 'rings' at a higher pitch than sterling silver. To understand the difference, simply spin a pre-1920 and a post 1920 and listen to the difference. However, I have no idea, apart from chemically, how to determine which alloy mix was used for the debased silver types.
Secondly, if the coins are very high grade, you may be able to examine the serifs of the 'Es'. Sterling silver coins have sharper points due to better metal flow, although the difference is tiny.