The most important thing to determine VA eligibility are discharge papers, also called a DD-214 or DD -two-fourteen. Look in your local paper in government or in the "V"s for Veterans Affairs or Veterans Administration. You should be able to find a VA Counselor who represents your county. It is their job to help you wade through these details. Be patient it takes time!
You may also find help and the counselors name by contacting your local VFW - Veterans of Foreign Wars. I was able to contact the in-take coordinator at a VA Nursing home in my home town regarding my uncle. If that doesn't work, try the nearest VA hospital. You'll also need a Power of Attorney to talk with people on behalf of your elder.
... and... check with the County Clerk's office where your FIL lived after returning from his military service. After WW2, many service men were told to "record" their discharge papers with the county clerk's office. We found a copy of my fathers paperwork there. This is important because years ago, each service member got a different ID #; they switched to social security #'s later but he might not have been in during that time.
I can't find my father inlaw's military records, he was in the Navy in the 40's. I know he used a different birthdate, not sure what he used. But his name is different than what's on his birth certificate, so have no clue what name he used.
You may also find help and the counselors name by contacting your local VFW - Veterans of Foreign Wars. I was able to contact the in-take coordinator at a VA Nursing home in my home town regarding my uncle. If that doesn't work, try the nearest VA hospital. You'll also need a Power of Attorney to talk with people on behalf of your elder.