I'd agree with the pople above who said to weigh it... but some other suggestions in case you don't have an accurate enough scale.
-look at it through a magnifying glass or loupe. Is there any flaking of the suurfaces, or do the date and mint mark show a buildup of "extra" material?
-hard to tell from that photo, but is it just oxidized or is it actually a silver color? Take a look at a pre-1964 silver dime/quarter/etc and compare the color.