Prj2EPSG is a simple service for converting well-known text projection information from .prj files into standard EPSG codes.see : OpenGeo – Prj2EPSG
“what is the EPSG number for Oregon State-Plane South?” and “how do I find the EPSG number for this shapefile?” One could search spatialreference.org, a site for understanding spatial reference systems.
My own answer used to be a fairly unhelpful set of directions for doing a text search of the PostGIS SPATIAL_REF_SYS table:
SELECT srid, srtext FROM spatial_ref_sys WHERE srtext ILIKE '%oregon%';
But today, I can provide a much simpler answer: Use prj2epsg.org. With prj2epsg.org, you can paste in full well-known text descriptions, you can type in shorter keyword searches, and you can even read a .prj file directly.