welcome to the club
look for loose boards, bricks, loose tiles, above the chimney flu, in the ash hole
FOR THE(fireplace),anything that looks ,odd or out of place
if all else fails
open it up,dig it up, pry it up, cut it up, i sound like forrest gump
treasure is everywhere people were, maybe
also look into books on the subject and do your homework
good luck
HH
Didn't you ask this question just a couple of days ago? Maybe providing more info about the specific place you are intending to search or the reason you believe there is treasure there will help people respond.
There are several good books on the subject. You may also try the cache forums farther down.
You can also use a black light, it will show where the walls are different from the rest, possible replaced or patched.
Go over everything carefully take your time, you don't want to miss anything. GL. HH