There is a way to check if a vehicle is properly taxed, and it's free and legal. Go to the Direct.gov website and put "vehicle check" in the search box. This will bring up a new page and from there it is relatively simple to navigate. All you need is the vehicle registration number and make and there's quite a lot of info about the car. There's also a wonderfully useful bit to check MoT info. But you need the reference number off the V5 for that. The really good bit is that it has all the MoT advisories listed to give you an idea about the vehicle's condition/service history.