How would one cheak if the water they are drinking is clean?

There are at least two kinds of clean when it comes to drinking water. Water must be save from bacteria/waste contamination AND it must be clean from toxic elements such as arsenic, mercury, and that kind of thing. If the water is from a municipality, they will have analytical data that can be accessed. If the water is from a spring or individual well, or from bottled water, I know of no way except to pay to have it tested. And that can run a bunch of money.

What if I used pH paper? If the pH read 7 would that necessarily mean that it is safe to drink?

no

ok thanks DrBob :)

The safest water to drink in the world more than likely will not read a pH of 7.0.

but it would be very close to 7 though. right?

Most drinking water is slightly acidic because it picks up CO2 from the air. Most of the drinking water I've tested ends up about pH of 5 or so. That's 100 times more acidic than if it tested pH of 7. But it isn't acid enough for most people to notice the difference.