WATER!
Having set off this Spring for an over nighter with what I thought was plenty of water, I found myself having to ration what I had, and ultimately suffering from dehydration. Had I had the knowledge to know what water around me was safe, and what wasn't, as well as the means to treat said water, I wouldn't have been in trouble.
Learn to find water and learn to treat it. From there, everything else will form.
Julia
Not really anything we can do to know 100% what water is safe and what isn't.
We can make a educated guess by estimating it's source and guessing if it's likely to have been contaminated by animals, human waste or chemicals, but clear well oxygenated water is just as likely to make you ill as a muddy puddle as far as we can see with our naked eye.
Some folks lead people to believe they have some sort of mystical power, that by looking at water with the naked eye and guessing to it's source they can tell if it's safe, in reality though it's nonsense they have no more idea than the rest of us.
You can swing the odds in your favour.
Avoid low level water sources especially those in, around or running through high population areas
Avoid low level water around farms
Filter, boil or chemically treat any water you take
None are a guarantee of safe water, you could take water from a stream 2000m above any sign of animal or human life, filter it, then boil it, then chemically treat it, but still some harmful chemical contamination may still be in it.
Unlikely but still possible.