The effectiveness of detector vans is an urban myth.
The only confirmed tactics is requiring stores selling TVs to collect the address of the buyer and knocking on the doors of people not paying the licence in the assumtion that they actually have a TV.
They drove a very obvious looking "detector van" around near our university halls for a few days, and a few days later every person that had a TV and no license got a letter, got scared, and paid up.
Of course, everyone that had no TV and no license also got a letter... and so did everyone that already had a TV license!
The only confirmed tactics is requiring stores selling TVs to collect the address of the buyer and knocking on the doors of people not paying the licence in the assumtion that they actually have a TV.