Here's how I do it. Find a current map of the place you're seeking. I locate the town in the index, find it on the map, and look carefully to determine the county in which it is currently located. It shows maps for each of the years in which the Federal population censuses were taken, complete with county boundaries as they were then overlaying the current county boundaries. The maps in this book are an invaluable reference resource.
Detailed articles describe specific record types and provide helpful information.
It is here that I determine specific years of origin for a county and can backtrack to previous counties or other governing entities as necessary. With these simple steps, you can quickly isolate the correct county in the right state in which to do your research. This is essential when writing for record copies and critical if you are planning a research trip to another place at some distance from your home. If you have taken the steps to determine the correct location for your research, I have to ask whether you are recording that information too. As a responsible genealogist or family historian, you are compiling the fruits of your research for others' reference.