It's really quite simple. Your finger is pricked with a tiny needle called a lancet, you squeeze out a drop of blood, and the dr. uses a monitor to read the concentration of glucose in your blood. Generally, you should fast for a few hours before this, as high carb intake immediately before the test may skew the numbers. I don't know the exact classification for diagnosis, but anything over 150 would seem to suggest you're diabetic.
Do you have any specific reason to believe you have diabetes? Excessive thirst, carb cravings, frequent urination, lethargy, metallic taste in mouth are all common symptoms of hyperglycemia (high blood sugar)
Frankly, there's no way you've had diabetes (Type 1, juvenile onset) for a few years and not been dx'
ed -- you would have gone into a DKA coma a long time ago.
I don't know your financial situation, but if it does turn out you have it, FIND A GOOD CLINIC. Treatment techniques vary, and you want to be on intensive insulin therapy at the very least, preferably a pump.
PM me if you have any questions, I'm an expert -- unfortunately
-James