Display Prices With Tax

Go to System → Settings → Checkout

Enable this setting if you're required to show prices that already include tax. If disabled, taxes will only be added to the order total on the checkout confirmation page.
When this setting is enabled, the text "Tax Inc." will be displayed next to the prices of taxable products on product listings and the product detail page.

Default Tax for Guests

By default, guest customers (those without a specified shipping address) will see product prices with the tax rate applied to your store's default address (Country and State). Please check your Tax and Location settings if this does not work for you.

The accuracy of this default tax calculation depends on the Default Store Tax Rate setting. Ensure this setting is configured correctly to reflect your store's location and applicable tax rules.