To see if your employees are paying extra tax for STSL you can find this information in two ways.
Employee Records
By going to individual employees and Payments, Tax.
You may have a joined Tax Table with both, or you may have a Secondary Tax with STSL.
HR Query
HR, User Defined Queries, select Employee Tax Tables.
Select your Query Parameters, OK.
When displayed, under the headings Tax Table and Secondary Tax, you will see which tax scale has been attached to each employee.
Note: If you have your Tax Tables + STSL under Tax Table your PAYG will show as one figure. If you have your Tax Table and a Secondary Tax with STSL your payslip will show the breakdown of both taxes.