Credit, criminal, eviction history.  

I am not being rude or offensive.

@Avery Goodman no offense to your attorney, but I might talk to someone else a bit more objective, make it clear you will not use their services only their advice and pay them for their time otherwise by the time you get 100 rentals you could be filing 50 different tax returns if each LLC owns only 2 SFRs.

A Real Estate attorney can facilitate transactions, but they probably won't help you with all the nitty gritty - either way you're going to pay for their time.I'm not trying to be a turd, so please don't take this as offensive - but you're probably missing a lot, and it sounds like you don't know what you don't know - and that's a really bad position to be in, especially in Real Estate.This is an expensive industry to be in and transactions always carry substantial liability. 

Most of the software out there will provide you with the applicant's comprehensive criminal, credit and eviction history.

@Bradley Coleman In order to save yourself a bunch of time you will want to set up minimum qualifications for credit, criminal activity, and eviction history. 

If all looks good credit and criminal-wise and they meet your rental criteria I would 100% rent to them.  

In you invest in D-class properties, well, you should think about having your own organized crime gang to back you up and not worry about legal recourse.I invest in what I'd call C-minus class properties in my area, kept from being D by several factors, chief among which is that Pittsburgh is certainly not criminal-friendly.

Besides doing a criminal background check and credit report, doing things like meeting them in-person, checking to see if their car is clean, and just getting to know them better are some ways to ensure you have a trusting tenant that is a good fit for you and your property.

(no offense).