qgis - Checking whether shapefile has overlapping polygons?

Using QGIS, what is the quickest way to check whether a shapefile has polygons that overlap each other?

I guess I could import it into PostGIS, then see whether:

SUM(ST_Area(geom)) 

is the same as:

SUM(ST_Area(ST_Union(geom)))

But that doesn't feel like it's likely to be the most efficient way to do it, especially since my shapefile has about 40k features.

Any ideas?

Answer

Without knowing what software you are using I can only answer if you use QGIS. Simply running the 'Topology Checker' tool will identify where all overlaps occur in a single layer