SirBax,
Here are the years to keep in mind, as far as silver content:
Nickels: 1942-1945, 35% silver content
Dimes: 1964 and older, 90% silver content
Quarters: 1964 and older, 90% silver content
Half dollars: 1964 and older, 90% silver content
Half dollars: 1965-1970, 40% silver content
Dollars: 1935 and older, 90% silver content
There are also older coins such as 20 cent pieces (1875 to 1878) that were 90% silver and 3 cent peices (1851 to 1873) that were 75% silver, as well as many proof cins and commemorative coins that were struck using silver through the years.
Here is a link to a great website for beginning detectorists and coin collectors. It gives just about any fact or statistic on coins that you could imagine:
http://www.coinfacts.com/
Hope that helps some!