I know what this means literally...a man wants to find evidence of cheating by his wife by feeling the sheets to see if the temperature indicates that someone else has been there. I'm trying to find an English equivalent and the only one I can think of is "to find the nigger in the woodpile". Of course, using the words "nigger" these days is a bit politically incorrect, although that would not stop me using it if there are no other possibilities. Are there any other suggestions?