He’s checking for intermediate frequency (IF). With radio, the signal is downconverted to a middle frequency to be amplified then down converted to actual audio. That intermediate frequency is emitted from the local receiver. It can be use to track down the receiver. In the UK, there is a TV tax that pays for the BBC, so he’s basically looking for tax evasion. The trade off is you get original Dr. Who or Top Gear without meddling from advertisers.