a hydrometer tests gravity....(
http://en.wikipedia.org/wiki/Hydrometer)it shows you essentially the density of the liquid which in brewing equates to disolved sugars. If you know how much you start with and what it finishes at (a lower #), you can derive alcohol content and figure out if thew yeast is done based on how low it will usually drop down. There pretty cheap:
Austin Homebrew Supply
Crawl through the cider threads and you should get some ideas...Welcome to HBT