Green’s Dictionary of Slang

to check carefully.

1883

Daily News 1 Aug. in Ware (1909) 62/2: It requires a stretch of fancy to picture forth an old-fashioned post-office, with clerks ‘candling’ the letters as if they were doubtful eggs. The conditions of a single letter were that it should be written ‘on one sheet.’ The letters were held up to the light to show whether they required a surcharge for an enclosure.