How to check which wire is switched live?
You figure out the live by using a voltmeter or voltage detector.  If a voltmeter you compare it to neutral (or earth will suffice for testing, if present.)

You figure out the switched-live by noting which of the  live wires is off with the switch off and dead with the switch missing.  

Generally you are stuck with the colors that twin-and-earth come in: blue and brown (or black and red, I assume you're going back to those colors after Brexit completes?)   So using a different color wire is not a luxury you have.  Thqt's why I own 10 colors of tape, but many installers do not bother especially if the usage is obvious.  

If power (always-live and neutral) are coming through the switch, the wire past the switch to the light will need neutral, so its usual neutral wire color must be used; the live color gets used for switched-live.  i.e. You will see two blues spliced together in the back of the box and two browns on the switch. 

If it is what we Yanks call a "switch loop", power comes to the light and only a single twin-and-earth goes to the switch. Neutral is not in the cable even though a neutral-colored wire is present.   One is always-live and one is switched-live, anyone's guess which.  The switch either connects them to each other, or does not.  Dimmers might care which is the always-live, but if it's wrong, they just wouldn't work.