When DH saw his GP and the GP suspected Type 2 diabetes, he had a knacky little gadget on his desk that pricked his finger and gave an instant reading, which when it showed a high reading was followed by a blood test.
The thing to realise is that while being overweight and leading a generally unhealthy lifestyle makes you more likely to get Type 2 diabetes, they are not a necessary qualifications to get it. I have a number of friends with Type 2 diabetes, who are fit and healthy with a good diet and have always been so.
The same applies to a lot of other problems from heart attacks and strokes to cancer. Certain lifestyles may increase your chances of getting any of these illnesses, but a good lifestyle does not mean you will not get them