I'm no slum lord but I have rented before. Officially speaking, and I'm sure you know this, most states have anti-discrimination laws when it comes to race/religion/etc. However you can do background checks which mostly useless but you make the renter pay anyway so why not. All it will reveal are 18+ official criminal charges. It's not like you are hiring some private eye to dig up dirt. I've also done credit checks on people and I highly recommend doing that.
Overall I did make money, but nothing to brag about. I think when it comes to real estate, and this is where I just am guessing a little based on experience, you either own lots of it or big commercial complexes(malls/office buildings). My experience is that owning just a few properties which make so-so profit is more hassle than it's worth. But consider that I have a different full time job. I suppose if I tripled the units I own and quit my job it might equivalent, but that's just not what I want to do with my life, even if the "sell them all and be rich in retirement" is better than my 401k and private investments now.
Anyway, stories aside, I probably make 15% annually. Obviously what you can charge is completely dependent on where you own.