fine-tooth comb

Short definition from Wiktionary

Noun

1. 1
   A comb with fine, closely spaced teeth, especially one used for removing head lice and their nits (eggs) from the hair; a nit comb.
2. 2
   A means of making a thorough search.

Verb

1. 1
   To comb or go through (hair, an animal's fur, etc.) with a comb having fine, closely spaced teeth.
2. 2
   To search (something or somewhere) meticulously.