I don't belive there is any hard and fast rule on the subject, but in my campaign, we let people know after they have rolled their save, fail or pass. That is not to say that we let them know "you were poisoned by dragon's bile". That sort of thing would require an alchemist to determine. On a successful save we would do something like "You feel a wave of nausea pass over you, sapping your strength, but you gather your strength and shrug it off."
I think it is safe to say that you are going to notice any poison, your save doesn't necesarily mean you sit there drinking cyanide and laughing. If one of my players were to do so and pass his save, I would probably rule that he promptly vomits it up there on the spot before it has a chance to take effect.
Ultimately it depends on the type of poison and the DM, but as a general rule, I would say yes, you would know.
Greythax
