On the zoom ring of your lens, it'll tell you what focal length you're at. On non-zoom ("prime") lenses, the lens should still tell you what focal length you're at.


Your Rebel body has a 1.6x "crop factor", so every lens you use has an effective focal length that's 1.6 times the focal length you see on the lens itself.


If shutter speed doesn't matter to the artistic/creative aspect of your pictures and you are shooting handheld, any particular focal length lens (combined with a particular camera) needs a certain shutter speed (or faster) to minimize/eliminate blurriness due to camera shake. A 50mm lens on your camera needs a shutter speed of 1/80th of a second or faster. A 200mm lens on your camera needs a shutter speed of 1/320th of a second. A 500mm lens on your camera needs a shutter speed of 1/800th of a second.


If there isn't enough light (aperture and ISO factor in here; we'll save their influences for other threads) to get shutter speeds as fast as the above guidelines, IS can help you out. The 70-200/2.8IS lens reportedly has a 3-stop IS unit. Therefore, at 70mm focal length you'd need a shutter speed of 1/14th of a second with IS, and at 200mm focal length you'd need 1/40th. For non-action shots, this can make or break your shots.


For action sports photography, 1/500th is a typical guideline for getting crisp action shots, so in those cases IS won't really matter. In a nutshell, if you aren't getting shutter speeds fast enough for stop action, you'd want to increase the ISO setting and/or open the aperture setting of the lens wider.


Consider getting a copy of Understanding Exposure by Bryan Peterson. It should explain the creative/artistic aspects of small/large apertures and slow/fast shutter speeds for you quite well.