You would have to set up some sort of service or polygon layer for the US counties. One possible solution (if you don't have your own US counties data) is to find a public WFS or WMS service. Here's one that doesn't contain counties, but is a good example of how you'd do it using OpenLayers.
Also helpful if by chance you're mapping fire stations in Medford, Oregon ;).
Here's a public US counties boundary WMS, but I couldn't get it to work.
Here's another example on the OpenLayers examples page: http://dev.openlayers.org/releases/OpenLayers-2.12/examples/getfeatureinfo-popup.html
If you're using your own data and don't want to make a WMS out of it, I think you could do something similar with KML. I'm not terribly sure of that, however.
Yet another possible implementation would involve PostGIS and some simple spatial queries, but I'm not sure whether that's even feasible for your setup.