Is my grandma diabetic or not?

ok so my grandma goes to a doctor and had her blood sugar tested and it was 400 so the doc said she was diabetic and that explains her symptoms but whenever she went to another doctor a month later they said she wasnt(thats why the medication hasnt worked on her) and me and my mother dont know what to believe and we cant talk to the doctor face to face because the doctors are in mexico. so how do we know for sure?