One of the first signs of an approaching shed is the snake having a milky belly. During the "in blue" phase, the snake's skin will look dull, and the eyes generally turn blue. In some morphs, it's hard to see the blue phase, but the eyes do generally cloud over, I find.
The snake should then shed roughly a week after the eyes have cleared up.
Watch to see if your snake is immersing itself in its water bowl. Sometimes snakes do that pre-shed, as extra moisture helps them in the process of wriggling out of their old skin.
You can on occasion miss the blue phase - ie, if the snake has just eaten, he/she can go blue while he's digesting in his hide. So don't be surprised if you wake up one morning to an unexpected shed!
A snake which is just starting to shed often begins to rub itself against something rough to start the skin peeling. So don't worry if you see your snake doing this.
Try to avoid touching your snake where possible, during a shed. While some snakes are very docile during a shed, others are quite agitated, and even aggressive, due to their minimal vision at this time. Offer food as normal during a shed - some snakes eat and some snakes don't.
If you see that your snake is having trouble shedding, or bits of skin are still left on him, you can help him out by raising the humidity. You can lightly mist the cage, or provide him with a damp hide - just fill a hide with some sphagnum moss. There are tons of helpful threads about shedding, so if you're at all concerned about anything, just run a search.
Hope that helped! Sorry for such a length reply, but the more info the better, imo. Good luck!