Q) When I clean my sensor, I worry about what would happen if I scratched it. How would I know, and what could I do about it?
A) You'll know if you scratch the lens covering your sensor because the scratch will be evident in every image, and it really will show up in skies or other constant tone/non-detailed areas. Look at the sensor with a SensorScope (www.delkin.com), and any flaws will be revealed. You're not likely to scratch a sensor cover, but it's possible. The most common way is by using a cleaning swab that is either too stiff or has a bit of debris on it. I've actually scratched a sensor cover on a Canon EOS-1Ds Mark II. I was testing a new swab, and it either had a piece of sand on it or wasn't soft enough. I use the term "sensor cover" because the sensor itself isn't exposed to the lens opening. There's a glass IR cutoff filter protecting the actual sensor.
How do you get a scratched sensor cover replaced? The camera manufacturer will do it, and another company that does IR conversions also can replace the sensor cover. You can get a price from both and then make your decision. Life Pixel (www.lifepixel.com) charges approximately $350 to replace a sensor cover. I don't have a cost from the individual camera manufacturers. They will be happy to give you an estimate.
A portion of a digital capture is shown here with red arrows pointing to an out-of-focus scratch on the glass filter covering the sensor. The image contrast has been enhanced to make the scratch more visible. The camera was a Canon EOS-1Ds Mark II with a 15mm fisheye lens attached. The ƒ-stop was set to ƒ/16.