Your question : How to know whether the USER or Person has 'Default Approval Role' or not?
Ans : You need to check how a Job assigned to a person in HR record linked through an approval groups assignement. In short, A person's is assigned to a JOB and this Job is linked to the Approval group assignment.
Table/view which you may be interested is : PO_POSITION_CONTROLS_ALL & PO_POSITIONS_VAL_V... Please have a look...