How thick is it?

The ice I walked on this morning was generally 7 to 12 inches thick. Except near shore, of course, where it thinned out to nothing at all.
How do I know that it is safe? I take great care, and I carry a long pole that serves as a poking stick. When I want to check any particular patch of ice, I give it a number of sharp whacks with the end of the pole. Most of the time, it barely makes a dent in the surface. Sometimes, just a few whacks will break through.
The colour of the ice gives some indication of strength. In this photo, the dark ice broke though with about 4 pokes. Yet, I could not poke a hole all the way through the paler ice a foot away. The ice at this spot was about 4 inches thick.

Colour alone is not an indicator of the quality of the ice. Some pale ice I walked on just a little closer to shore had a 'soft' feel as I stepped on it. It whispered to me, a little creeeek. I did not poke a hole there...no point making it weaker while I was standing on it! I went back to the quiet ice, and found a better place to walk back to the land.