checking custom field on record create

Our sales team needs to create orders for products & services. Whenever a line item is created a custom field is populated in each line item with that items' "category".

I am trying to create a custom script that will check to see if any of the line items in any given opportunity are of a specific category. If this is true (i.e. if items of category "PS" are present) then I want to prevent the sales rep from creating an estimate or a sales order until a 'approved' box is checked on the opportunity. (the sales reps don't see the "approved" check box - only the PS team does).