There is no query available out of the box. This option gets automatically checked for a task, one it has one or more subtasks. The one option if you are looking for the actual query behind this field is enabling PPM Action trace and evaluating the action trace. (See KB000009353 for how to enable this)
Example of the steps to see this value checked:
1. Create 2 new tasks in a project in PPM
2. In the PPM Gantt, add the 'Has Subtasks' to the list view if not there already.
3. Make Task 2 a subtask to Task 1 by clicking the check box next to Task 2 and then clicking the 'Indent' icon.
4. You will now see that Task 1 has a check box in the 'Has Subtasks' field.