Their are two Sales Tax Reports located in the Sales Reports in the Admin Panel.
To get there click on the Admin button in the top bar.
Click on the Sales Section and you'll find the Sales Tax report at the bottom of the list.
Use the Filters at the top of the page to set the timeframe and sales agents for the report.
Sales Tax on Accrued Sales displays the tax for orders that have been completed (rarely used by our customers).
Sales Tax on Cash Sales shows the sales tax you've actually collected (95% of our customers use this report).