My reading of the film is that Anakin may have been a famous pilot (best starpilot in the galaxy) but he lost much of his fame when he decided to join the conservative monastic order which was the Jedi. This is just speculation, but I figure that Obi-wan could very well have first noticed Anakin's Force sensitivity by simply watching him fly his ship.
As for the Force, I suspect that it may be terminology used only by Jedi and Sith, and other religious orders have other names for this mystical energy field. Heck, if I was part of the Sith order, I would come up with some other name for it than "The Dark Side of the Force". What a mouthful, as well as implying that it is evil! The Emperor could simply be using that language on Luke because that's what the Jedi call it, and using the Sith names for things would just get confusing.