Ask to see the V5C vehicle registration certificate (‘log book’). Make sure it has a ‘DVL’ watermark, and the serial number isn’t between BG8229501 to BG9999030, or BI2305501 to BI2800000. If it is, the V5C might be stolen - call the police as soon as it’s safe to. Most new V5Cs are now red but some older ones may not be - to check the V5C is the latest issued use the DVLA vehicle enquiry service.
Make sure the details in the log book match the details you’ve been given.