You need the person NAME, SERVICE NUMBER and SS NUMBER if you have them.
To help get this information, you can go to the STATE where he/she enlisted.By US law the STATE is responsible to keep track of their troop.
The information was kept on a 5X7 card later put on MICROFILM.it reflected the person name, date of birth, address, last of Kin, Rank, Muster, in and out dates,Honorable discharge or not,It also shows the engagements, Medals and ribbons received.
Though the records are not accurate the attempts to do so were honorable.
Contact the State where he/she enlisted and check in the STATE ARCHIVES.Military documents are kept since the state became organized.A small cost might be there.
Once information is found you can get the "MORNING REPORTS" by writing the DIVISION HEADQUARTERS this will tell you more information of that member and what they did.