How to determine Live switch wire

Follow given advice. Return if "ghost voltages" exist. FFS...

How much should the op spend on a "proper multimeter" to do the job properly?

Rather begs the question, why do you have one? given your op...

I am changing an extraction fan with timer. The old fan has 2 red and a black wire but no indication as to what one is live switch, how do I determine what one of the reds is live switch, with the power switch off at mains.

Insufficient advice. Doesn't allow for eliminating false positives ("ghost voltages").

Easy way - look at which is connected to the existing switched live terminal on the fan, then mark it with tape before disconnecting. :-)

I would be tempted to make a guess knowing there's a 50% likelihood of it being right!

Yes it was maybe something that they should have done before taking the old unit out using a meter with a probe assuming of course te on situation is controllable and is not some function of a thermostat, but then I think if one is live and one not you have your answer if everything is cold at the time. Brian

I agree however I did mark one of the reds so when I took the old fan down I could then match it up with the identification markings on the fan, only then did I discover the old fan is marked N and L but nothing for the 3rd wire. I did take a picture before disconnecting but that doesnt solve it due to lack of identification markings.

Stick a bulb across, one of the lives and neutral. Then put the power back on.

Either the bulb will be controlled by the light switch or not (permanently on).

I assume using my cheap multimeter I could do the same and check for voltage?

Yes. Look for changes when you 'modulate' the light switch.

Yes, but not with a "cheap multimeter" - those ones typically pay zero attention to proper mains isolation despite their self-declared CAT ratings and have no MOVs etc.

Not too cheap, some don't really like mains even though they have a big AC range. They don't have enough insulation to be safe. I certainly wouldn't use the £1.99 ones I bought on mains if I had to touch them.

You don't have to spend a fortune. I use one of these for anything to do with the mains:

You don't touch any metal parts of a "cheap multimeter" as they are all plastic apart from the metal parts of the probes.

For this application a non contact volt stick will work well:

Leads are a suspect as well.

I was wondering what was constantly draining our car battery, and what current it was - expecting a few 10's of milliamps drawn.

Erm no.

With a series connection I found I had rather more. Amazon's finest 99p test leads have a nifty built in detection feature.

Smoke....

(I now have a decent clamp meter that does DC)

Which is why I bought a Fluke 179.