I often heard this method is reliable and I am also using it. From experience I can say it works. Also if you search the web you will find articles like for example this and this.
Also available are color charts to get an visual impression, see e.g.:
Besides that personally I know that I am not prone to get headache. So when I start getting slight headache I am aware it could be due to dehydration. Often it helps to drink some water which is the cheapest and healthiest medication against headache.
From the link above you see there are a lot of symptons to be aware:
- Muscle cramps
- Dry Skin
- Dry mouth/lips
- High body temperature
If you get one or several symptons or your physical perception changes, you should be aware it could be dehydration. Because, as I already said, the medication has no negative effects at all you should simply use it just in case.
Still keep in mind that you could get the symptons because of other reasons/diseases too and if nothing changes after you got hydrated you should start investigating. One common issue is AMS (altitude mountain sickness) which has a lot of similar symptons than dehydration. So simply drinking water might not help you if you are getting headaches and start feeling dizzy/fatigue at higher elevation.