As Captain2 has indicated, entering at least part of a clue into the search box (at the top right of a page) should find any threads which include the same phrase. (It's best to put double quotation marks around your search string).
The OP is asking how to find out if a clue has been asked before. That is done without anyone else on AB knowing about it. Without the clue no-one can 'swoop' anywhere/on anyone.