From the Settings (Admin Control Panel -> Taxes -> Settings) menu:
If checked, enables tax for site fees.
Select the tax that will be applied to site fees:
If checked, allows sellers to apply tax on their listings.
The selected tax will be automatically applied to site fees based on user location.
To see if the tax is applied to site fees go to Account History (Home -> Members Area ->My Account -> Account History) and select the corresponding invoice:
See below how to find taxes on a receipt: