Well... one way is real simple.
I believe Wisconsin tried it.
If memory serves me correctly, the moment they said "If you want receive help, you must come in and if you are able to work, you will work for the State until you get a job... all of a sudden, the rolls were reduced substantially.
But it is pretty easy to find out. Have them come in and be evaluated. Have them look for jobs (with some sort to proof). If after one week they come back and say, "Well, I wanted to go find a job but I had a head ache the whole week and couldn't go. I didn't go to the doctor for a doctor's excuse because my foot started hurting... you can pretty much figure it out.
I remember as I was working for a grocery chain some people would come paying with food stamps. I would put their groceries in their nice Lincoln, Continental and then admire their $60 nail painting job. Do you think they really needed the food help?
Others would come in a car that was old with children. No frills and no painted nails,.
do you think it was pretty obvious who was milking the system? Or do you believe everyone is an honest worker?