Here is a possible way to track

1 With what you describe I would assume that the Purchase users have access to sales orders.

2. For those purchase users who would be creating POs, enable the option 'Lock confirmed sales'. These additional technical options are on the user access form visible only while on developer mode

3. This would now enable the lock button for these users on the sales order

4. Once the PO is created for a sales order, instruct the users to 'lock' the sales order.

5. It is now possible to filter SO by locked field. (In your case 'locked is not set')

If you have studio it is also possible to add the locked field to the list view

Alternate to the able process could be to track on some other fields i.e set a tag or value in the other information tab.. source document with PO # etc)