You need a pH tester. The one from Aquarium Pharmaceuticals is good. I'm sure it's at your LFS.
However, you should know that, although you will be told by a fish store that fish "need" this pH or that pH, your fish will adjust, just fine, to just about any pH you are likely to have in the tank. What will upset the fish is sudden swings in the pH.
About the worst thing you can do when it comes to pH is buy chemicals, like pH Up or pH Down, to adjust them, because what they do is temporarily raise or lower the pH. Then, the tanks natural ecosystem puts it back where it was, and you put more chemicals in, etc. That puts the tank on a pH roller coaster that really IS damaging to the fish.
If you need to adjust your tank's pH, do it naturally, by changing the tanks environment, NOT with chemicals.
Some crushed coral in the substrate or filter will raise both the pH AND the buffer (which keeps the pH up). Some driftwood in the tank will probably lower it. Those are the simplest ways.
Personally, I try to make my pH match the pH in my tap. That way I can do large water changes without worrying about too much of a pH swing.