For purchasing turnkey rentals with tenants, how do you deal with not seeing the inside of the property?

[Alex Madison]

I figure you would need to establish a GREAT deal of trust before doing so. I'm interested in purchasing a few SFRs, and there are some companies out there who have renovated properties with tenants in place. Even if the company was completely reputable, I would still want some reassurance that the property doesn't have any major problems. How much due diligence can be done in a case such as this?