You can view your vouchers from the My Vouchers page.
Another way to check the balance of a voucher is to enter it in the Promo Code box on the Order Page, and then click apply. Simply click on the Remove option next to the code once it's applied if you wish to enter another code. If you do not wish to place an order, you can simply close the browser tab for the shopping cart. No order will be placed unless you click on the Place Order button on the final page.