Is a Wet Spot in The Ceiling From Toilet Plumping?
You are going to have to do some testing.   The first thing to do is assess how likely it is that you could have water in the bathroom from spillage.   If water isn't running into the toilet area - from people showering or washing hands/whatever - then you might as well pull the toilet up.

To pull the toilet you will have a bolt on each side (undo the nuts) and a water supply line (turn off water and disconnect by unscrewing.   Once water is turned off flush the toilet to get rid of as much excess water and then pull it up.   

Now you will need to assess what things look like and you may need to provide pictures to us.  There are several reasons a toilet would leak.   The first and easiest thing to do is dump a bunch of whatever down the toilet pipe.   It is highly unlikely that you have a leak there but this is super easy to check.   Do this first.   

Next assess how the toilet flange looks and how high it is.   It could simply be that the toilet release is a bit too high and that over time the flange plus wax ring degraded enough to cause a slight gap.   This is most likely your issue.   

If the flange is worn, brittle, or cut then replace it.   If the flange is good but too low you will need to replace it with a flange that sits higher.   This is an easy job and just takes a few minutes.   

If the flange looks OK height-wise and what fixes 90% of toilet leaks is replacing the wax ring.   Since you have a leak I would suggest an extra thick ring.   

(I didn't mention everything but covered what should help 95+% to make it easier - let me know if you have more specifics questions or pictures)