With his handy VISOR, would Geordi be able to detect Changelings masquerading as humans, Klingons, or even fog? Or could the Changelings still fool him?
Geordi's visor is the product of known Federation technology. Unlike Data, who was created by a mad scientist using (at least in part) technology of his own invention, there is no indication that the technology in Geordi's visor could not be reproduced, adapted or improvised in any of the myriad ways that the Federation is so very good at.
During the Dominion war, detecting Changelings was a Starfleet priority. If there existed any known technology with the potential to detect them directly (without the need for a blood sample) it would undoubtedly have been developed regardless of expense, and we would have seen it in use, at least as a prototype.