The eviction is on their record, but without being able to find them (short of hiring a private detective), the attorney said the judgment we won was vacated.

Also, my last 2 potential tenants have had evictions within the past 5 years and it did not detect it. 

In some jurisdictions the licensed real estate brokers have raised enough stink that wholesalers are being "harassed" by the legal authorities for operating with a real estate license. ...Don Konipol - I think I detected a typo in the above quoted passage; I believe you intended to write this:"In some jurisdictions the licensed real estate brokers have raised enough stink that wholesalers are being "harassed" by the legal authorities for operating without a real estate license."

Well, this was an exciting and interesting short exercise in REI detective work.

More troublesome it is an owner occupied unit (not just someone's bad renters that may move on)...So if I got another fourplex, I would put on my detective hat and actually research adjoining and nearby properties, and their owners, very well.

My cleaning man & maintenance man BOTH cannot detect any unpleasant odors in these places, but they're there.

I know they sell those socket extension with motion DETECTION for about $20, is this my solution?

We use Nest Cams at our house and they send us notifications via text or email if it detects any movement.  

This is a combination of your own detective work, talking to neighbors etc. and the information you can gather online. 

Also, I could use some help detecting problems I may run into with my proposed plan.