Well, I know there is a "under the influence of a controlled substance" crime. I'm going to assume they determine this with drug tests, though I would think unless you were being SUPER out of it, you were being belligerent or disruptive in a public situation (not very likely on weed) they won't have probable cause, and would require your consent for the test. So basically, if you don't have anything on you, just try and act normal and refuse drug tests. If you're driving and you refuse however, I think you can loose your liscence, but I'm not sure.