yank your cranks off and turn the bottom bracket spindle by hand, if its bad, youll feel it. It will turn rough, or have some play in it. Dont expect it to spin freely though, there will be a bit of resistance from the seals that is normal just so long as its smooth. Of course if its creaking the bottom bracket tube could be a bit out of round and the bottom bracket no longer threads in tight, that would mean a new frame...
Another thought if it is the bottom bracket creaking in the frame add a little teflon tape to the threads.