Frodo's Barrow Dilemma

[Inziladun]

Originally Posted by Gordis

In LOTR there are no cases of Gandalf not seeing the wearer of the One.

'Hullo!' said Bilbo. 'I wondered if you would turn up.' I am glad to find you visible,' replied the wizard, sitting down in a chair, 'I wanted to catch you and have a few final words.'

FOTR A Long-Expected Party
(emphasis added)

If Gandalf was capable of seeing the wearer of the Ring, why would he make such a statement? How would he know, by appearance, whether Bilbo wore the Ring or not?