It is proper in genealogical research to place the county with city and town locations. For example, “…he was born in Madisonville, Hopkins County, KY.” Why? Because over time county boundaries changed and while an ancestor may have changed his address, his actual location or home may have remained the same. As complete a description as possible is thus useful.
How do you determine the then County boundary? Well, Google may help in some cases, but the best answer comes from the book The Handybook for Genealogist by Everton Publishing.
The former web site for Everton and indeed their telephone numbers do not answer when searched or dialed today. There are suggestions the company has bankrupted and other suggestions they may be bought by another group. But for the moment I am unable to raise them.
So what do I suggest? This book mentioned above is a wonderfully useful help in performing genealogical research. This book provides a history of each state, a listing of every county in each state, including when formed, formed from which other county, telephone number, what type of records are available, and where those records are located (County Clerk, Records Management, etc.) In addition the book includes State and County maps, migration trails and early canals and railroad maps. You will not sit down and read this book, but you will refer to it dozens of times a year for as long as your research continues.
Now, this book was in its 11th edition earlier this year. I would guess at this point that will be its last edition. I own an 8th edition, copyrighted in 1991 and in its 4th printing in 1994. I find it is entirely adequate for my continuing needs. Frankly, history does not change, but if telephone numbers and record locations have changed, a query will determine that fact easily. So this book will be useful under any conditions.
My suggestion. Buy an old one on eBay or bookstore or wherever you can. As soon as you can. It may – I repeat may – never be available in updated form in the future from the publisher. It is too important a resource not to be in your library.