You can usually see a crack in the bottom, not always. I will get knocked for asking, but did you use a wax ring with a tail piece? Many guys say they will make a toilet leak, I always use one when I add the extenders, many of the guys hate them and I'm sure will post against what I said. I knew a guy that dumped some red dye in his toilet and flushed and and picked it up right after, he said you could tell where the leak was coming from. Have you read the wax ring question thread?