Typical stories that involve Caesar Ciphers will give the Detective encoded text, and some clue as to how far to shift each letter. Here's an example:

Please forgive this mysterious way of contacting you, but this is a matter of the utmost urgency and secrecy. You must locate me before something disastrous happens! To find out where you must go, there is little you must know. I can give you this rhyming clue, letters you must shift by ?

The Detective would then be given the nonsense letters RCTM.

To decode this they'd have to recgonize it as a Caesar Cipher, and then realize that the only number that rhymes with clue is two, and then shift the letters backwards by two. The R becomes P, the C becomes A, the T becomes R, and the M becomes K. So the decoded word is PARK.