The American Diabetes Association (ADA) recommends testing guidelines for diabetes in certain people:

Those at any age with a body mass index greater than 25 and those with additional risk factors like high blood pressure, sedentary lifestyle, high cholesterol, or heart disease. Testing is also recommended for people older than 45 years, people with close relatives who have diabetes, women with polycystic ovarian syndrome, women who have delivered a baby who was heavier than nine pounds, and women with gestational diabetes.

Diabetes can be diagnosed by testing the amount of sugar in the blood, either after fasting or after an oral glucose tolerance test, or by measuring the percent glycated hemoglobin (hemoglobin A1c or HbA1c) within the blood. HbA1c is a representation of the average blood sugar levels for the past two to three months, and results of 6.5% or higher on two tests are indicative of diabetes. HbA1c is less sensitive than a fasting glucose or an oral glucose tolerance test. It is important to note that HbA1c is an indirect measurement of average blood glucose and other factors may affect HbA1c independent of average blood glucose, such as age, race/ethnicity, and anemia/ hemoglobinopathies.

Source:
https://www.mayoclinic.org/diseases-conditions/diabetes/diagnosis-treatment/drc-20371451