My neighbor's bathroom ceiling has a leak. I live in a condo (they were converted from apartments). The neighbor’s bathroom is right under my bathroom, she stated that I must have a leak. I checked all the plumbing that is visible and my toilet is not clogged and has not been clogged there is also not any water on the floor. I don't know where to look for the problem. What could be causing this leak?

While I am not a plumber, I have or "had" the same issue with the toilet I'm trying to now change upstairs. Go figure right before we were going to remodel it started leaking. Luckily I could find the leak because its above our basement.

My guess is that the wax ring the toilet sits on to create a seal has gone bad or maybe the toilet is cracked at the base. I'm not sure if you have to call anyone to work on it since you live in a condo, but I'd recommend just pulling the toilet and taking a look.

What you'll need to do...
Purchase a new wax toilet ring before you do this (regardless of what you find), they are say 5 bucks from the hardware store, then shut the water off to the toilet and unscrew that connection so you can totally remove the toilet. The toilet will be bolted to a metal ring in the floor with two bolts, one on either side holding it down.

You may need to pop off a couple of caps hiding those bolts. Once undone just pull the toilet off the ground. I recommend trying to empty the tank and bowl of water and taking off the tank lid first. Then maybe even get a couple of towels handy to get the little bit of excess water that will spash out.

Once the toilet is up you'll see the old ring and can inspect the bottom of your toilet. You may find the ring totally destroyed or a crack in the toilet. If the ring looked really good and sealed and the toilet has no cracks you may have to pull the toilet mount off the ground.

I'd guess the ring or toilt is the problem and a new ring or new toilet will solve the problem. The worst you could do is find out its neither but put it back together with the new ring anyway until you can spend more time to dig a big further or get somebody else to come in and take a look at it.