If the cap fits

If the cap fits (wear it)  (British, American & Australian) 

also If the hat/shoe fits (wear it). (American)

something that you say to tell someone that if they are guilty of something bad, they should accept criticism 

• Look, I didn't say who was to blame for this mess - but if the cap fits, wear it.
  

... http://idioms.thefreedictionary.com/CAP