The only place to confirm if you received an HOV toll discount is your monthly toll statement.
The purpose of the History screen is to display toll transactions for which GoCarma confirmed the occupancy status in your vehicle.
The History screen does not confirm whether you receive an HOV toll discount. That is confirmed by the toll operator and is dependent on the status of your toll account.
For example, if your toll account is not in good standing, it's possible you won't receive the discount, even if GoCarma confirmed the vehicle as HOV.