For instance: must qualify on their own, no co-signers, no criminal record, no evictions, good credit, stable income that can be verified, verified previous 2 landlords that are not relatives, etc.

She qualifies for Section 8 but her monthly income is far less than 3x the monthly rent.I know Section 8 should guarantee rent gets paid on time each month, but I worry about her having the money to actually care for the property and yard while she's there.No past evictions and clean criminal background.Has anyone had a similar situation?

Tenants are allowed guests and friends , unless its criminal activity not much you can do 

Why would a criminal crack the glass when kicking the door in would be easier and faster?

Have you done a criminal history check?

It says it checks criminal history, eviction records, credit scores, etc.

I ask what issues we will find when we check their credit and criminal record.

But I need some help figuring out what categories to even look at.I'm going to start with the basics: are they employed, how much do they make, what is their credit score, have they ever been evicted, do they have a criminal background, etc.  

I am saying the opposite, and is what I do, that I say no to almost every applicant with an eviction, terrible credit or a past of criminal convictions.

The existing lease had a waiver of notice (good thing) and another clause to "follow all local, state, and federal laws"The girlfriend (who IS on the lease) has a guilty plea entered in late November... and as a lesson-learned when inspecting leases during due-diligence I've fount the PA Judicial portal to be a Godsend.https://ujsportal.pacourts.us/Default.aspxThis little gem lets me research by party name (tenant) in Criminal (statewide) and also local matters by magistrates office.