You might consider having a plumber run a camera through the lines to make sure there aren’t any other plumbing surprises awaiting you.

If the building is secured (ie front door locked), then install cameras to monitor the area where packages are left. 

Exterior motion-activated lights (three with wifi cameras -internet not included) are on all four sides of building. 

The doorbells are Ring (https://shop.ring.com/pages/doorbell-cameras).

And let's be honest, people can get pretty strategic with the camera when they're trying to move a property.

If you just need a camera or 2 or a motion sensor to alert you on your phone, if someone breaks in, are there options?

The interwebs will only give you part of the story in a world where Photoshop use and wide-angle real estate camera lenses are widespread.

The physical work part involves cleaning out units after people leave (and leave trash), mowing the grass, maintaining the fence and security cameras if you have them, over-locking unpaid units, conducting auctions to enforce your lien. 

The only method obvious to me is setting up a camera. 

, truck to load it (later I bought a trailer), good camera (iphone can work too), Drone (nothing shoots land better than aerial pics and vids), marketing budget, rubber boots ( good for swamps and cold weather snow), rain gear, bug proof clothing, Sawyers brand repellent, a good hat, marketing material, letter head, computer, printer, filing cabinet, RE license, certifications.