During the course of a surveillance, the vehicle driven by the subject may offer more information than just the registration details.
For example, a quick look inside the vehicle may reveal his occupation, place of employment, or places where he frequently parks his car if you see unpaid parking tickets inside.
If you suspect the subject is involved in criminal activity or insurance fraud, then pay for a report from CARFAX and CarProof and get a history search on the vehicle identification number (VIN). The history for the VIN will reveal any liens and state if the car was involved in past accidents or if it has been marked as a salvage, re-built or non-repairable vehicle. Also run the VIN at the Canadian Police Information Centre website under the stolen vehicle section.

