Less than 4 hours a day is considered shade, 4 to 6 hours is part shade, and over 6 hours is full sun. I don't know of any tool to measure it, just the gardener's observations. On a sunny day when you're home, set a timer to check the sun every hour from morning till evening. Make up a map or chart of your yard and note when the sun is on each area. At the end of the day, you'll be able to tell what "type" of sun/shade ratio you have in each area of the yard.

Of course, to be really accurate, you need to do that every few months, as the sun moves thru the seasons. A spot that's shady in summer can be sunny in the winter when the leaves fall off trees and plants go dormant. I'm always amazed at how much it changes through the year.