http://www.muschealth.com/psychiatry/professionals/lab/frequentlyaskedquestionsUrineEthylglucuronide.htm
The test will look for levels above 100 ng/ml. It'll be sensitive to booze in your system for around 96 hours. So, if you had a 6-pack and they test you, you'll be above 100.
500 is the "I drank last night" level.
If you have to show up for anti-drinking class a few times a week, there really isn't a good way to continue to drink and have to take this test