Following up on dmsnyder's suggestion, I'm creating an entry on pH meters. I few years back, I did some research in order to purchase an inexpensive meter to track the pH of my fish pond, which was under reconstruction. Around the same time, I was trying to get a sourdough going, and I felt this would help. I ended up with a Hanna Instruments meter, which is available on Amazon for $31.15:
This meter allows one to calibrate at two separate pH's, and ideally you would do this with one above, and one below the value you are trying to measure. For example, when I was playing with sourdough starters, I calibrated with pH buffers of 7.01 and 4.01. Obviously you need to purchase (or make) the buffers. pH buffers are available online, although I purchased mine from a local university chemistry stores.
I've been very pleased with my meter, although other customers on Amazon have not been as fortunate. In any case, I feel that it is one of those tools that should be in every home. (I forgot to mention that it is a very useful tool to keep your pH sensitive plants happy).