I've heard the paper testers don't work very well. You could start off without one and just watch your plants if you're an experienced grower. Have you ever grown in soil before? Hydro is a bit harder, because you don't have the soil to act as a buffer.

If you're a fast reader, you can compare the pictures between your grow and someone having problems. Nice, healthy green leaves are what you want. The public libraries have hydoponic books.