When the cycle starts (as visible to us), you might notice the snake's colors will be a bit darker and less "colorful" than normal. About two days later the eyes will cloud over and your snake will be what's called "blue". After being blue for 2 or 3 days the snake will clear up again and then shed. The whole process can take anywhere from 5 - 9 days depending on the age of your snake and how soon you notice the snake's cycle starting.
It can be a bit difficult to notice in certain morphs though.
Hope that helps a bit.