Ohms. (also called resistance/continuity)
With the power to the circuit off.
First, put the meter on an ohms range, if the probes do not touch each other, the reading will be whatever the meter shows for no connection (inf, infinity, a big 1 or what ever - remember this )
Then touch the probes togther, the reading should become a low number (0, 1 or 2 sort of thing.) That is the resistance of the probes and meter internals.
Now you know what your meter says for 'well connected' and 'not connected' respectivly... (an intemediate number is the resitance of the object - when bored measure a light bulb for a laugh.) A switch should be only either a very low resitance when on, or practically infinite when off.
Now hold the probes between the metal ends of the wires that you hope might go to the switch.
If correct, when a colleague operates the switch, (or you on your own if you like running up and down steps or have either a small room or arms like a gorrilla), the resistance between the pair of wires will change between the two states (connected and not connected) as the switch is operated (connected for switch is on, unsurprisingly).
If no change when switch is operated you have the wrong pair of wires... roll the dice and try again. (or the switch is short of course)
Thats it.
Some meters have a 'beep' setting, so that you know the connection has been made without looking - can be handy as you can leave the meter probes jammed in the terminals while you operate the switch from afar.
Please remember to remove all meters and tidy up before putting the power back on.
come back if this makes no sense after you have had a play...
PS only works with proper switches - clever dimmers and things need to be replaced with a normal switch temporarily.