Add [wpgv-check-voucher-balance] to a page. Customers enter their voucher code and see the original value, the remaining balance, the expiry date and the payment status. Customers of a WooCommerce shop additionally find the same information under My Account → Check Voucher Balance.
For partial redemptions outside WooCommerce (e.g. a booking system) use [wpgv_partial_redeem]: the customer or your staff enters the code and the amount to deduct.