There are Internet Service Providers (ISP) who have been known to throttle your bandwidth if you are a high volume user. This can be because they think you may be breaking their Terms of Service (TOS) agreement by downloading a lot of music or video.
There are sites out there that will check the speed, but this is a one time test. If you want to really keep an eye on your upload and download speed, you should take a look at Net Speed Monitor. The app on your Windows computer will check the speed of your connection at your wireless card or hardwired connection. This is the techy equivalent of checking for horsepower at the wheels of a car instead of at the powertrain.
After you download Net Speed Monitor (download link below), you will be asked if you want the stats to be displayed in the taskbar. I’d say this is the easiest way to see the real-time stats so select yes.