fine-tooth comb
fine-tooth comb {n. phr.}
Great care; careful attention so as not to miss anything.
The police searched the scene of the crime with a fine-tooth comb for clues.
My room is so clean you couldn't find dirt if you went over it with a fine-tooth comb.
Compare: LEAVE NO STONE UNTURNED.
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Source: A Dictionary of American Idioms