Things like a website, an email at your domain and credible social accounts (even small things like a profile pic and bio) can show you are a legitimate business.

You should ask questions of your insurance agent, broker, or “surplus line” broker or contact the California Department of Insurance at the toll-free number 1-800-927-4357 or internet website www.insurance.ca.gov.

Know this (found on every inspection website):  You should be concerned about cracks in your home if they are large, appear suddenly, or are located in certain areas.

Would that be, for example, if the PM says, "I manage a duplex for John Smith in Turlock," then I call John as a reference, ask John a few questions, and then ask John the address of his duplex so I can later look it up on the county website and verify if he is the owner?

I actually have a wholesale oppurtunity on a multi-family Four-plex that I just promoted on the "Real Estate Classified" section of the Bigger Pockets website!

You can look on websites like https://www.corporatehousingbyowner.com/search/United-States... and https://www.furnishedfinder.com to see what similar houses are rented for.

Quote from @Chris Lo: Which platforms/websites do landlords use to find the rent rate, how long the rental properties have been on the market, and all the details about other rental properties?

The state website can also be very informative. 

@Chris SeveneyI agree it is not the most professional website I have seen.

With setting up LLCs would someone go to their Secretary of State website and do this?