How can you tell what the pH balance is in a fish tank?

Go to any good, and most not so good, pet stores, and you will be able to buy a water testing kit relitively cheaply.

From there it is a simple matter of following the instructions and adjusting your water to suit.

If you are looking to maintain relatively high pH levels and water hardness, ie, for African cichlids, limestone scenery may be a good alternative.

You can also get a Buffer Block from most pet stores. They are designed to hold the water at around pH7.