Is it not possible that, given that he was wearing his own Ring, Gandalf could tell when someone put on the One? I believe Galadriel also knew that Frodo had worn the Ring without asking him.
I think I always assumed that Gandalf somehow could just "tell", "magically" if you will, that Frodo had worn it, or had guessed.
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"Since the evening of that day we have journeyed from the shadow of Tol Brandir."
"On foot?" cried Éomer.
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