Create a saved search that will show Invoices that are not flagged as taxable ( Taxable box unchecked)

The system only has the criteria for Taxable (Line) to represent if the transaction (part or full) is taxable.  As alternate solution, customize the Sales Tax Analysis Report:
 
1. Navigate to Reports > Sales tax US > Sales Tax Analysis.
Note that this report will give you the Taxable Sales and the Non-Taxable Sales part of your transactions (Invoice and Cash Sales).
 
2. Click on Customize button.
 
3. On the Report Builder page, click on Filters link.
4. On the Search field (left side of the page), type in Non Taxable Sales and click on the Search button. 
5. Once the field is pulled up, click on Non Taxable Sales.
- Note that the Non Taxable Sales line will automatically appear on the right side of the same page.
 
6. Set: Filter = 'not equal to 0.00'
7. Click on Done.
8. Click on Preview button.
 
The system now shows you the transactions where the Taxable checkbox is not marked.