There are three PostGIS functions for checking validity. The first one is the ST_IsValid function. It behaves exactly like ST_IsSimple: it accepts a geometry and returns a Boolean indicating whether the input is valid or not.

SELECT * FROM data_import.ne_110m_land WHERE ST_IsValid(geom) =  FALSE; 

But what's exactly wrong with returned geometries? PostGIS can return the details of a validity check. Those can be returned in two variants: as a human readable summary or a compound type that includes the summary and location in geometry format. The first variant is named ST_IsValidReason:

SELECT ST_IsValidReason(geom) FROM data_import.ne_110m_land WHERE ST_IsValid(geom) =  FALSE;  

It returns a summary of test results:

                              st_isvalidreason                      
------------------------------------------------------------
 Ring Self-intersection[-132.710007884431 54.0400093154234]

The second variant is named ST_IsValidDetail, and returns a valid_detail compound type (similar to a geometry dump) containing three fields:

• valid, of a Boolean type, indicating whether the geometry is valid
• reason, of a text type, indicating the reason for its invalidity (if any)
• location, of a geometry type, indicating the location at which the error was encountered

This variant is useful when making validation reports.

For example:

    SELECT ST_IsValidDetail(geom) FROM data_import.ne_110m_land WHERE ST_IsValid(geom) =  FALSE;
NOTICE:  Ring Self-intersection at or near point -132.71000788443121 54.040009315423447
                            st_isvaliddetail                             
-------------------------------------------------------------------------
 (f,"Ring Self-intersection",010100000018717462B89660C0B8A376061F054B40)  

The following is used to explode it into separate columns:

SELECT (ST_IsValidDetail(geom)).location, (ST_IsValidDetail(geom)).reason FROM data_import.ne_110m_land WHERE ST_IsValid(geom) =  FALSE; 

NOTICE:  Ring Self-intersection at or near point -132.71000788443121 54.040009315423447
                  location                  |         reason        
--------------------------------------------+------------------------
 010100000018717462B89660C0B8A376061F054B40 | Ring Self-intersection