How do I recognise a 21 pin DCC socket?

Model railway 21-pin decoder sockets are easy to recognise. They are usually located on top of a circuit board inside the model and feature 21 pins approximately 5mm in length. The pins are arranged in two parallel rows, one row of 10 and another of 11.

If the pins are not immediately obvious when you open up your locomotive, it is likely because a blanking plug is already in the socket. A blanking plug looks like a smaller version of a decoder, but with less circuitry printed on it.