Very difficult, foxy, as Karl has said.
Even if you do trace the likely source, you'll still have to deal with it.
In the "good old days", you just had to pull a couple of floorboards and investigate. Today though, bathrooms probably have chipboard floors and vinyl covering.
Worse still, tiles or hardwood flooring. Either way, investigation means a good deal of disruption to the bathroom. Re-laying vinyl, and all the rest.
Certainly have a plumber check it over first. Then, although you won't like this, my way would be to simply cut an inspection hole in the kitchen ceiling containing the stain.
The bigger "trapdoor" the better. Then you can get your head up there and shine a torch over most of the bathroom floor void, and probably find the leak. Leaking joints are quite easy to find then, and repair.
Believe me, it's a doddle to nail the same piece of plasterboard back up, and fill the saw-cuts. Even if you have to cut yet another access hole, it's still a much easier job than having to put the bathroom floor back together.