When the eyes start to cloud up - when they are beginning to go "blue".

That is the beginning of a shed cycle when the eyes start to cloud over.

also, their skin will turn kinda greyish...my anery sand boas eyes are too small to see easily if they are in blue, so i use this option for when his skin noticeably dulls. However, for the rest of my snakes, I use it for when their eyes go blue. Just my . 02 :)

Yes, it's the beginning of the shed cycle but can look different or similar depending on the species of snakes.  My anaconda will develop a silvery hue, or my albino burmese turns extremely pink, or other snakes seem to get a grey tint.  That's why it's known as "turning opaque". 

As the old skin begins to seperate from the new skin, a layer of special oil that is produced by the snake develops in between the 2 layers.  This is also why the eye cap clouds over, as that is also a modified scale.  During this time the eyesight is severely compromised or lacking all-together.  After a few days, the snake will return back to a normalish color for a couple of days, then sheds.  The reason why snakes look so "shiny" or "slimy" after a shed is because they still have that remaining oil on their skin, but obviously comes off very quickly once the snake is shed. 

i agree with everything everyone , especially geegmasta, has said.

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