We are not aware of any obligation of a dealer or private seller to inform you that a vehicle attracts the premium rate of VED, but we would hope that if they do not volunteer this information we would strongly recommend asking them. You can also use the Honest John Road Tax Checker here - www.honestjohn.co.uk/car-tax/ - simply enter the registration and it will indicate 'higher rate' if this applies, and the DVLA's Vehicle Enquiry Service here - vehicleenquiry.service.gov.uk/?locale=en - will indicate until when the additional rate applies.