Silicone is used for a variety of home improvement projects. Sometimes this sealant is intentionally used on glass (to seal aquariums, for example) and sometimes it ends up on glass by accident. You may need to then remove it, say, before resealing a gap or, if it's there by mistake, simply for aesthetics. Removing this kind of sealant may not seem easy, but with the right tools and chemicals, it can be done.
Put on protective gloves and eyewear. Ventilate the room by opening doors and windows.
Place the utility knife blade parallel to the glass and carefully slide the blade beneath the silicone, as close to the glass as possible. If you are unable to do this, start at the top of the silicone layer and slice through the silicone layer by layer. The more you get off the glass this way, the less time the other removal steps will take.
Apply silicone remover to the remaining silicone. Leave it there for the amount of time specified on the label (it varies by product but should take less than five minutes if you've removed most of the silicone).
Gently scrape off the remaining residue with a putty knife. The silicone solvent should have loosened it enough to make this very simple.
Put warm water and a teaspoon of liquid soap on a sponge. Wipe off the whole area and then rinse it with warm water.
You'll find silicone removal chemicals at hardware stores, home improvement retailers and some automotive shops. If the spot of silicone is very small to begin with, skip the initial scraping and cutting steps and just apply the silicone solvent.
Be very careful when cutting through the silicone with the utility knife. Keep your other hand out of the way and always cut away from yourself. In addition, it is very easy to scratch the glass accidentally this way---if you rush, the knife can slip. Most solvents give off toxic fumes. Even in well-ventilated areas, this can be a problem. Take a fresh-air break whenever necessary.