Megan, normally, cities have a permit database that can be easily acccessed by anyone.
You also may want to consider a more important issue:
There are two different valuations that come into play when we talk about these situations. The first is the value placed on such a property by current market demand. The other valuation comes from a licensed appraiser. Given the uncertainty of unpermitted work, an appraiser will not be able to place any value on such work.
For example, say a Buyer places a $500K offer on a home that has $50K in unpermitted remodeling. In this simple case the appraiser would assign a $450K value, which would require the Buyer to come to the escrow table with an additional $50K as the lender would only provide financing on the appraised value of $450K. Another issue is that of safety. Do you really know that the work was done properly? In short, buying a home with unpermitted remodeling is gambling with your financial and personal safety.
Best Regards, Steve