Basically they take a blood sample and have a look at various chemicals and they can tell if you are kicking the arse out of the bottle.
No they can't.
First of all they need to appreciate the difference between being a heavy drinker and being an alcoholic. Both are problems, but for different reasons and in different ways.
As mentioned, one of the so-called identifiers for heavy drinking is Gamma GT. The 'norms' for an adult male are anywhere between 8 and 70 IU/L (International units/litre) depending on whom you ask and in what country. Mine is consistently around 100, has been as high as 200, and I have always been a very light drinker, in fact my doctor says that by some standards I am almost teetotal, probably consuming about 5 units a week. I know people who drink that at lunchtime and then double it in an evening, consistently and one of my friends who drinks more every lunchtime than I do in a week, has a Gamma GT of 24.