How 2 verify additional living space was built w/o legal issue?

[Sang Han]

Dear Fellow RE investors and experts, 

I would like to have your guidance here. I am looking for a property which has following information in its description. 

"Family room addition with half bath". 

I asked the listing agent if the seller followed the proper processes (drawing-permit-licensed GC-approved and so on) when he built this area. However, according to the agent, the owner purchased the property about 25 years ago, and the addition was already there even before the current owner purchased the property so that the agent would not know anything. 

Can any of you know how I can verify if everything is correctly done? Thank you very much. 

Sang