Anna & Anno. Thanks for the input. The sign was a violation of city code (and, I'm sure it would have been left up if I hadn't said anything to tenant.). I have a plan now, to do the walk through of the rental. I should be able, also, to tell if adult son is actually living there. At the same time I'll mention the van The walk through should tell me a lot.
Please research your state and local laws. Each state may be different with regard to family residents. I am confident that in NY there is little you can do with immediately family moves in, unless they violate your lease for unauthorized guests (such as an increase per person living in the unit to cover wear and tear), or they violate HUD housing requirements for the number of occupants per the unit size. That does not mean that as a landlord you don't have the right to at least get information on any adult tenant living in the unit.