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Zoom right in on something close at maximum focal length to amplify all of your wobbles. Hold it one-handed if necessary to make it wobble Then half press the shutter and the IS benefit should be very obvious in the viewfinder.
Of course, the camera must be on for IS to work. Make sure your camera has a well charged battery, just in case.
Make sure it is switched on (the switch is on the lens) and make sure it is set to mode 1 or 2 and that none of the switches are half-way. Flick the switches to and from the other settings to make sure they are working properly.
The IS will stop soon after you release the shutter button - like within a couple of seconds or so. (Too quick for my liking but you cannot change it even with the timing tweaks on a 1-series camera).
IS can be quiet or noisy. Unless you have used it before you may not hear it until you are tuned in to what it sounds like, especially if you are in a noisy area. Sometimes there is click at start up, and sometimes not. Varies a bit from lens to lens.
Try same lens on another camera if possible, and also try another IS lens on your camera if possible. Both combinations should work.
Without IS the chances are that at 200mm a shutter speed of 1/40 seconds will give a blurry shot hand-held, but with IS it should be sharp. Again, focus on something close to make it obvious.