Aside from the other excellent evidence, I think the simple line, “I have passed the test” shows she was really tempted. If she was just making a point, there's no test to pass.
TheThis question may arise if someone wonders why, if she was tempted then, she would not have been tempted as soon as Frodo came within her reach. I would answer that by saying that, while she was never tempted to take it by force, which would break Frodo's mind and betray the entire enterprise, the situation markedly changes when he truly, freely offers to give it to her. Then she does see it as feasible to claim the ring, and it is a test she passes when she refuses.