The other way to tell is if it sound too good to be true, it is. Our brokerage just recently helped to catch some of these dirt bags.
Your best bet is to do as Frank suggests or call a property manager that actually rents for owners.
Every agent has access to all of the rentals that are posted on the MLS and there is a move at this time to get everyone to post their legal rentals on the MLS so we can be of assistance to renters. Watch for a better public-facing site for rentals in the near future at HiCentral.com this is the local real estate board's website.
That said, there are literally thousands of illegal rentals that are for the most part ok, besides the illegal part, but make sure you ask questions about fire walls, electrical safety, separate electrical safety if you think it may not be a legal rental. It can be unsafe, especially if local joe handyman did it himself.....
Caveat Emptor for rentals,
Team Leader for TheOahuAgents.com