Well, it's relatively easy, I think. Once you are in the system, the social workers get access to your criminal record. If you have a DUI or had been busted for taking drugs in the past, tough luck, you are in the top of the list of the people who get surprise drug tests. Then, I'd rule out the old people and focus on the 18-30 age group initially. I'd do random tests, but respecting the same rule as above. In time, you should be able to clean the welfare system of leeches. I'd do the same with abusive people (wife beaters especially), people who refused more than 2 job offers and so on. There are solutions, though the politicians need votes and therefore, will protect certain... groups.