fine–tooth comb 
/ˈfaɪnˌtu:θ-/ noun
plural ∼ combs /ˈfaɪnˌtu:θ-/ noun [count] : a comb with many small teeth
go over/through (something) with a fine-tooth comb
: to examine something very closely
▪ The police are going over the evidence with a fine-tooth comb.
▪ The police are going over the evidence with a fine-tooth comb.
