But then I thought: how does it know? It's not like the head is wired in. It's deliberately detachable for cleaning. There are no electrical contacts. Could it be there is a RFID chip in the head and a RFID reader in the tooth brush?
Sure enough the Braun Oral-B 5000 features a RFID reader and RFID chips in the heads – a toothbrush area wireless network!
A quick google of "13MHz RFID" does reveal that 13.56MHz is popular RFID frequency. Unfortunately decoding this signal seems involved. There is an open hardware sniffer project here: http://www.openpcd.org. However I don't really have the time or incentive to hack my toothbrush right now.
An oscilloscope trace from the coil wrapped around the toothbrush head. A strong signal of about 75ns period is observed for about 1 second after the brush is switched on.