It really depends on the snake, some show few signs of an approaching shed but most appear dull or dark in colour, their belly becomes cloudy as do their eyes. Also some but not all refuse food. Usually it is quite obvious.
what boa and mfreud have said except one of my coastal gets dark blue eyes instead of a milky blue colour. i'd say all snakes are different i have 1 snake i can only tell its about to shed when its colours are really dull and boring