I've plugged Shapefiles into QGIS, which was able to discern the corresponding EPSG numbers. The .prj files also contain enough information to figure out proje

I've plugged Shapefiles into QGIS, which was able to discern the corresponding EPSG numbers. The .prj files also contain enough information to figure out projection IDs, and various tools exist to manipulate and extract information from those files. It looks like that webservice is a wrapper around one or many of those tools.