Stain on ceiling below second floor bathroom
My condo was built in 1984-85.
The builders cut many corners with the plumbing.
For instance, the downstairs bathroom toilet didn't have a flange (discovered when replacing the vinyl tile). No kidding.
Now, it appears that I may have a serious problem with the upstairs bathroom as well.
A very pale stain has been slowly (several months) developing on the popcorn ceiling below the vicinity of the one-piece fiberglass tub or toilet drains. Just now, I noticed that the stain has a slightly darker spot in one place. The stain is about three feet long by a couple of inches wide.
I'm envisioning the bathroom floor being torn up, having to sell my first born to pay for it, etc.
What is the path of general troubleshooting for something like this?
Should I plan on hundreds of dollars, or thousands of dollars for the plumbing portion of the repairs?
Is it better from a plumbing standpoint to tear up the floor or ceiling should either be necessary?
Thanks for your help.