The 10% toll discount shows separately on your account. If you log into your Tag account and click the ‘View Transactions’ button, you will see a list of all of the crossings you have made. On this screen it will simply show the toll tarif your vehicle comes under – £1.50 for a car (for example).
The discount is shown on a separate tab on this screen – ‘Charges/Deductions’. You will see an amount for the discount (£0.15 for a car, £1.20 for an HGV, etc) with a date and time that corresponds with your crossing.