The vast majority of people wear their watches on their non-dominant hand, so it's usually a pretty good indicator. Yes there are one or two people who don't do this, however in general it is correct. There are other ways of telling if you really need to, although it's accurate enough for practical purposes. You probably wouldn't exclude someone from an investigation solely based on this, but it's still a useful clue.
Remember that in real-world investigations, often you aren't going to start with evidence that definitively proves something one way or the other. Often you only have relatively tenuous evidence to go off, but often it's enough to get started.