The property is zoned as a 2-unit but have 3 independent units to potentially generate rent.

I’ve worked a bunch on other large projects and inspections  (building, zoning, plumbing, and electrical) and have had no issues in PA and NJ. 

You'll probably want to do some more digging on these tenants before accepting them.6 adults and 4 kids in a 4/2 home is a red flag itself.

Does anyone know of a reliable site or resource to check zoning laws for Airbnbs or STRs in general?

There are several properties that either are zoned correctly or would need rezoning, but priced out of our range, so hadn't made any offers yet, there.

. - Restricted by zoning.

Although I've been investing in and improving multifamily properties since 2000, I decided to go back to school and get a Bachelor's Degree in Construction Management from BCIT (I already have my Red Seal in Carpentry).

Ask about any unforeseen challenges that have come up (there always are, so not all challenges are a red flag), and overall let them know you're an involved LP who likes to be kept up to date on things

Combine this with poor ADU valuations (especially in single family zoned areas) result in hands off ADU additions costing significantly more than the value they add.  

I have called 311 with a couple of code enforcement/zoning questions, and they have always been able to put me in touch with someone who could answer my question.