DCC Track Monitor
This simple device shows you whether you have DCC power and signal on your track. It can either be permanently wired in place at some suitable point on the layout, or built with flying leads attached to the ends of the 270 ohm resistors, and used to check different sections of the track as required. Any convenient type of light-emitting diode can be used, but both resistors should have 0.5 watt rating.
Both LEDs will light when DCC is present on the track, but only one LED will be lit, depending on polarity, if straight DC is present.