They will then probably do two things. They will do a "signal test" on your modem. While useful this isn't the holy grail of whether your speed is where it should be. They will also probably have you go to their proprietary speed test as well such as this http://speedtest.nyc.rr.com/ Here again useful but this isn't necessarily a real world indication of where your true connections speed it.
What I would recommend doing is going to a couple of speed test sites that are pretty consistent and can give you a gauge of where you stand during the "good times" as well as the slow times. This way when you feel your speed is lagging you have a pretty good idea of where you speed should be when times are good as well as bad. And if your speed has never been what your ISP says it should be you will have more information at your disposal.
The first site is http://www.speakeasy.net/speedtest/ This is a great site because it is very easy plus it gives several geographic locations that should be close to anybody in the continental United States. I would test the two closest locations to yourself and compare.
The next site is http://speedtest.net/ This one is much more comprehensive and will automatically detect your geographic location. What I like about speedtest.net is that it gives quite a bit of analytic information about your speedtest. Below you will find a photo of the speedtest.net result.
Hopefully this helps you on your way. Good luck. :)
