This is Green Deanes' method for determining the safe edibilty of a plant :
I.T.E.M. :
"First, it means (I)dentify the plant beyond doubt. Next, make sure it is growing or fruiting or otherwise available at the right (T)ime of year. Third is checking out the (E)nvironment. This involves two things. One is making sure it is growing in the right place. The other is making sure the plant is getting clean water and is not in polluted soil. And then, what is the proper (M)ethod of preparation. I’m sure you can do that. I.T.E.M. You can rearrange the letters to spell T.I.M.E. if you like as long as you always remember to apply the four steps"
Here is the link :
http://www.eattheweeds.com/foraging/