Set Sales Tax on Default on Customers and Orders:

Utilities Menu

J. system setup 

J. sales tax / invoicing options...

B. sales tax on Labor

changing the setting will prompt "would you like to update all customers"  select YES

next the system will prompt "would you like to update all open orders" select YES or NO depending on the timing of this change.

Verify the Tax per Order or to change the tax per sale:

On the sale go to [F6] options and T. Taxable Status to make any changes.

Tax Status per Item:

To double check the tax status per item simply view the items in the default labor and parts view and over to the right of the total on each item is a "Tax" column this can be edited on each item as needed