Do you have a glass tank with a screen lid? If so, is the lid covered with plexi-glass or cardboard lined with aluminum foil? What substrate are you using?
I live in MA, and keeping humidity up in the winter months is a pain in the butt. So, I feel your pain. When any of my snakes go into shed I always add some extra plastic tubs/deli cups/tupperwares, etc...filled with sphagnum moss or wet paper towels. It clutters the enclosure, but helps with the humidity and (knock on wood) I've had only perfect sheds to this point. My Cal King, Django is probably going to shed today (fingers crossed) or tomorrow.

A pink belly is usually the first thing I notice when my BP is about to go into a shed. Usually a pink belly and dulled looking skin are my first tip-offs.
I have seen my snakes rubbing their noses on different things in their enclosure once in blue and prior to the actual shed.

Also, not to burst your bubble, but if you're measuring the shed to get a length of your snake, it is a very inaccurate representation of the length of the snake. Sheds tend to stretch out and are not the length of the actual animal. Sorry.