Point the hour hand towards the sun, then true south will be halfway between the hour hand and the 12-o'clock mark.
Only if your watch shows solar time which is normally not the case.
If you don't adjust for DST and what part of your timezone you are in it will be off by something like 30 degrees. If you need better than that the adjustments are not very hard (The 80/20 in most places being to just subtract one hour from what the clock shows because of DST).
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