Select the link for the Manual Coupon yourself. If it has been used, you will see the message ‘Coupon usage limit has been reached’ at the top of the checkout page.
If the link has not been redeemed, you will see the message “Coupon code applied successfully. The <Manual type> has been added to your cart!”.
Make sure not to check-out the order, otherwise you will be redeeming the voucher yourself.