When Gandalf "figured out" The Ring?

[Attalus]

I think the point about Gandalf not worrying overmuch about Bilbo's Ring despite its powers of invisibility is well-reasoned. The Gwaith-i-MÃ*rdain would regard a ring that conferred invisibility and nothing else as gravely defective, since they wanted to preserve "all things unstained" and invisibility would fall under the rubric of "deceits" so scorned by Galadriel. Conceivably, such a ring might be offered to a mortal for services rendered. It was indeed when he saw that Bilbo was not aging according to the nature of Hobbits that his deeper fears would be aroused.