fine-tooth comb, to go over with a

go over something with a fine-tooth comb

 and search something with a fine-tooth comb; go through something with a fine-tooth comb
Fig. to search through something very carefully. I can't find my calculus book. I went over the whole place with a fine-tooth comb. I searched this place with a fine-tooth comb and didn't find my ring.
See also: comb, go, over

fine-tooth comb, to go over with a

To search for or investigate with scrupulous care. Although combs have been known since the time of ancient Egypt, and presumably some had finer teeth than others, the term “fine-tooth comb” dates only from the first half of the nineteenth century. The transfer of combing out nits to other kinds of search or investigation took place only in the late nineteenth century.
See also: go, over