ST_SRID — Returns the spatial reference identifier of the raster as defined in spatial_ref_sys table.
integer ST_SRID(raster  rast);
Returns the spatial reference identifier of the raster object as defined in the spatial_ref_sys table.
                 
               | 
              |
| 
                 From PostGIS 2.0+ the srid of a non-georeferenced raster/geometry is 0 instead of the prior -1.  | 
            
SELECT ST_SRID(rast) As srid
FROM dummy_rast WHERE rid=1;
srid
----------------
0