Check Geometry Validity

It is accessed through:

Tools > Check Geometry Validity...

It is used to check if the layer has invalid geometries and why it is invalid. The main reasons are: 'self-intersection', 'ring self-intersection', 'nested shells', 'Invalid geometry type found (MULTIPOLYGONZ)'.
  1. Select the layer to be analyzed.
  2. Click the Verify button and wait for a list of all invalid geometries and the reason.
  3. Double-click the list in the FID column to locate the invalid geometry on the map. A small green square will appear in the CENTER of the map over the invalid geometry. Zoom the CENTER of the map and click again on the FID column to visualize the invalid geometry. 
HINT: The error message 'Invalid geometry type found (<geomType>)'  usually affect all geometries when the layer has more than one geometry type.  In this case, the <geomType> will be different to the one shown in Properties(*) and the vector processing options will not work properly.

Use the Vector Processing Make Layer Valid menu option to correct invalid geometries and set the <geomType> to the most comprehensive one.
For example, if the layer mixes POLYGON and MULTIPOLYGON, MULTIPOLYGON will be set as the output geometry type.

(*) Righ-click over the Layer and choose Properties