In addition to the skin turning cloudy, one sign that I've found to be reliable is to look for a nose that's been wetted slightly by constant licking. If you see that, you're usually within a couple hours of the time.
My guess is that they lick their noses to soften the skin even more just before the start of the shed.
They will often begin rubbing their head, neck, and jaw against the hide walls to get things started. Seems like the longest part of the shed is getting the skin back to the shoulders. Once there, the rest seems to go pretty quickly. If you see them constantly moving about and turning around in their hide then it might be this process of rubbing the initial skin off the head or neck.
Usually takes mine somewhere in the range of 45 minutes to an hour to complete. A good 2/3 of the time is spent around the head and neck. Once they get good purchase on the old skin it seems to come off the forearms, body, hindlegs, and tail relatively easily. Maintaining a good humid hide has probably helped my geckos to shed easily.
After the process is done, I'll give them a bit to rest and then try to check their toes, tail, and head for unshed skin.