Tips on how to check if this guest house in ADU was built with permits?
Very easy to resolve.  Have seller put in writing that the ADU was legally permitted when built.  He has put you on notice with &quot;seller is 'convinced' that it was legally built.&quot;  If you accept that statement, you are making the purchase &quot;as is&quot; based on what he believes, not factual evidence.
Only if he puts it in writing that the ADU is legally built would he bear responsibility for misleading you if you later have an issue with the &quot;ADU&quot;.
Ways to validate if ADU was built with a permit.
Go to the planning department and request a copy of the blueprint. The permits would not have been issued without the drawings. Also seller, claims the ADU is on the plan, so ask the seller for the plan showing the ADU. 
Edited based on additional information: 
The &quot;guest house&quot; was originally built as a &quot;workshop&quot;.  Later converted into livable space, “ADU”,  conversion most likely done without a permit.   So your underlying questions has been answer.  
Per your added comment, “If my understanding is correct, then Workshop does not count towards &quot;legal livable area&quot;, but yet they still included it.”.
I do not believe there is a California law that regulates how a home's square footage must be calculated.  Livable space needs to meet certain requirements, i.e., heat, ceiling height, etc.”,   The bottom line is you are purchasing the “ADU” feature “As Is”. Whether you believe the seller should include it as part of the &quot;livable&quot; space is your opinion.  Even if the seller removes the footage as part of the livable space, the price would still be the same.