Unfortunately, primary prevention (stopping it happening), seems to be blurring with secondary prevention (stopping complications once it's happened).
The WHO stuff on diagnosis is probably the best guide, although, obviously local authorities will have their own rules.
Random, (or indeed fasting) glucose levels aren't that helpful, as Radgirl say's, HBA1c is probably the best indicator of future problems.
I'm not sure what the stuff about histamine from Winnerhofer has to do with things, I'd be happy to help him sift through the evidence behind his assertions if he care's to post it.