I'd suggest something along the line of Health Inspectors; think of those we have today: while yes, there's the rare incidence of e-coli or something, it's generally pretty safe and appealing to eat in a restaurant. Slaves have a "licensing fee" to pay for these routine (and at times, surprise) checkups, which I imagine aren't the warm cuddly sort of inspection. With blood, urine and fecal samples, and a thorough bodily examination the inspectors determine how much stress the slave has gone through, how badly theyve been beaten, etc. It may not be perfectly realistic, but it's not that beyond either.

There would also be checks. Logically, people with slaves consider them a huge investment, and just as one wouldn't crash a porshe just for fun one wouldn't injure a slave, either. There would also be some sort of "underground railroad" attempting to free slaves, particularly those that were abused badly, and of course some sort of watchdog groups: like Greenpeace, but for slaves. Finally, it would be legal for slaves to take action to prevent greivous injury -- and that would include maiming or killing their owner in any circumstances that get deadly.

None of these are perfect, naturally, but as the author you could simply declare them "very effective."