To start go to the NGS Tool Kit here:
exactly... we also have a reverse geocoder on the site under TOOLS in the navigation...
If you want to do it long hand, try these...
I just got a Autocad macro (vba project) working that will convert State Plane Coordinates to Latitude and Longitude. Click a point in the drawing and it converts the SPCS of that point to Lat/Long and then labels the values at the point you clicked. You can also Enter Lat/Long and a point is plotted and labeled.
You must have the corpscon_v6.dll in C:\program files\Corpscon6 along with the folders containing geoid, nadcon, vertcon data files. These are installed with the free download from http://www.agc.army.mil/corpscon/
I tested in Autocad 2004 and 2013. The form is setup for my local SPCS zones (1601 and 1602), but it can be modified for any zone(s).
email me if interested.