Create an empty result table using this format: (pkey_column NUMBER, result VARCHAR2(10)).
Call the SDO_GEOM.VALIDATE_LAYER procedure. It is important to specify a "commit interval," which tells you the number of geometries that require validation before the application will give you a "DONE" message. This will help you monitor the progress of the validation while it is still ongoing.
Look at the "RESULT" column as the procedure operates. If a particular row has invalid geometry in it, this column will have a code indicating a particular error. By consulting your troubleshooting guide, you can find out the specific problem at that point in the column.