I want to replace a pendant light with a track holding 4 halogen lights. It is the last light in a loop in circuit so there are only two cables visable, each with a red and black wire and no earths. I understand one of these cables is the circuit and the other is to the light switch and should be marked as such, but neither has any identification at all. Is there a simple foolproof method to establish which cable is to the  light switch? Any hints would be very welcome.

if you have a continuety tester, or a torch that will do

kill the power at the fuse board
use the main switch unless you have a meter that you can be sure of telling you its dead(test it on a known live source to prove the unit -  you should make sure you have only one main switch and there arnt any hiding)

then get the two cables, seperate their cores out
connect your meter (or between the battery and the bulb if your using a torch) and operate/ get somone to operate the switch.
if the meter shows continuity when the switch is operated, and then dies when its disconnected then thats the cable.

ss

The light circuit has no earth afraid to say. The house was built around 40 years ago and still has the original wiring. All the light fittings are plastic as is the new track which does not have an earth. Thanks to all you guys for your input which has helped me get the job done