Military records can be very interesting to explore. I do have the history on my Great Grandfather and I know that he only had to fill out the draft card and never went into the military. He was a husband, father, and farmer in Coon Rapids, MN
A total of 24.3 million American men registered for the draft (about a quarter of the population), of which about 2.8
million were drafted. Another 2 million men volunteered or otherwise
enlisted (those who had already volunteered did not have to fill out
draft registration cards). In all, about 4.3 million men were actually
mobilized, and about 350,000 were casualties, of which about 126,000
were killed. http://wiki.answers.com/Q/Was_there_a_military_draft_in_World_War_1
The best way to find out if you have a veteran in your family is of course to talk to your family. That isn't always available to most of us. Search engines are great at finding the missing pieces.