If its a few year old, you can check the mileage online using a smartphone / ipad / laptop etc at
https://www.gov.uk/check-mot-status-vehicleYou will need the previous mot certificate number (which anyone/garage who is selling shouldnt mind you looking at BEFORE you buy it) and the Reg No...
Not only does it let you know the list of all previous mot milage, but what it failed on and what advisories it had... Handy to know if somone has botched a job on it!
Regards,
Jason.