I think the easiest solution would be a visual indicator like Stevez said.
If that won't work, I would look for a way to fit a proximity switch on the blade, and either make it switch on a light just before the winch runs out of cable, or better yet, make it shut down the winch.
The trouble is anything electrical you make is going to need to be rugged and sealed up very well. I live in Minnesota, so I know what kind of abuse a snow plow sees
Cold, extream vibration, packed snow, salty sloppy corrosive snow ect.
I have an old bulldozer with a cable lift blade. It dosn't have anything to prevent this same problem. You can't see the pulleys either. You just learn how high you can raise the blade in relation to the ground, else it has enough power to snap a 1/2 cable and drop its 4 ton blade.