Let’s make one thing clear: Water color and cleanliness does not correlate. At all. A yellowish water source can be caused by natural organic matter such as peat and be perfectly fine, maybe even add a lovely taste. A fully transparent water source can contain farm fertilizer that will destroy your stomach permanently. Don’t be fooled by color.
If you do want to assess the water, then look for this:
Size does matter: A large river is generally safer than a small creek, as any potential pollutants are diluted proportionally.
Any cattle? Cows share a dumb characteristic with humans: they shit in their water and deliberately pollute their own environment. Don’t drink untreated water if cows are around. Or any cattle, really.
Altitude: The higher you go, the less mammals and the less risk of a carcass or excrements. Above the tree line and you’re generally safe.
These parameters are all indicative and not any guarantee.
You should, theoretically, carry a water treatment system with you all the time. That said, on the PCT, it is fairly common to drink the water directly from the creeks in the Cascades and the Sierras. You will be very happy with a treatment-system when you reach the stagnant ponds of Oregon and the cattle-troughs of Southern California.
A filter is the most common water treatment, as it doesn’t leave a chlorine aftertaste. However, it’s also larger and heavier than the droplets and requires a bit of pumping work.
The tablets or droplets are tiny and lightweight but most leave some aftertaste: Either a chlorine taste or it masks it with, say, raspberry. Some claim to have no aftertaste, but this is disputed by most hikers I’ve met.