Here's a study page on my website that will give some information on what button backs look like and particularly some struck during the Civil War period, if you find one with an RMDC backmark, that's a pretty good sign you have a Civil War period button, with just a few exceptions, here's that link:
http://www.vintagebuttons.net/shanks.html And, here's another study page showing a variety of historical and uniform buttons of interest:
http://www.vintagebuttons.net/uniformed.html Get the "Albert book" Peg mentions above, it's a good start to study US uniform buttons. Warren Tice has a few out and the Big Book of Buttons has an overview of foreign and US uniform buttons worth studying.