What is the test for determining whether a police officer had reasonable grounds to arrest a driver for possession of a small baggie of marijuana?

"Here, it is clear that this court is not called upon to decide the reasonableness of a belief on the part of the officer that petitioner was a co-possessor of the baggie, because the record is devoid of even the slightest suggestion that the officer entertained such a belief. The record discloses that petitioner had been arrested for 'reckless driving' (not for possession of the baggie), and that petitioner had been booked for 'reckless driving' (not for possession of the baggie). Even the most tortuous interpretation of the facts before us fails to establish that the arresting officer ever suspicioned that petitioner had been in either personal or joint possession of the baggie. Since there is no evidence that the officer envisioned that petitioner possessed the baggie, it becomes illogical for us to analyze whether such a contemplation would have been reasonable had it been entertained; since there is no evidence that the officer ever contemplated (or if he did, formed a belief) that petitioner possessed the baggie, the officer lacked lawful grounds to arrest for possession of the baggie, and, absent a lawful arrest, the subsequent booking procedure and search were illegal." (Agar v. Superior Court, supra, 21 Cal.App.3d at p. 30, 98 Cal.Rptr. 148.)