1. If it's in the Mustard family, edible.
2. If it's in the Mint family and smells minty, edible. (My new rule)
3. If it's a seaweed, edible.
4. If it's in the Lettuce tribe (dandelion-like flower with all rays with five "teeth")
Henbit is an edible mint that doesn't smell minty, by the way.
All the plants i found yesterday were Eurasian, and so far only one, groundsel, is not edible.
I leave you with yesterday's oxalis:
This is also posted at http://elainegrey.dreamwidth.org/549041.html .