There are many books and field guides on this topic. Some of them are probably at your local library. Check out the book "Edible and Useful Plants of California" by Charlotte Bringle Clarke for starters.
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Yes. You can, too, by investing time and effort to research the subject. In fact, you might even get a Master's or Doctoral thesis out of it.
It depends what you mean by 'edible'.
If you mean "it won't kill you or give you horrible cramps and vomiting to eat a handful of these leaves" then pretty much anything is edible in that sense. If you mean, "people used to eat it after applying the appropriate processing techniques", you have a smaller list. If you mean, "it is a good source of nutrition", that list is very short and mostly confined to agricultural plants.