You will need a strong utility knife to cut out the garage ceiling drywall. Only cut to the size of the area where the ceiling is wet because if you cut too big an area, later you will have problem repairing your drywall ceiling. Then look into the ceiling and see where the source of water leak is coming from. In my own case, my bathroom was located directly above the garage ceiling. So i figured it was the toilet potty that was leaking water into the garage ceiling below, every single time that i flushed the toilet potty. Now having figured the source of the water leak, i did open up the toilet potty myself and lifted the potty to see why the water was leaking out.
I found out that the toilet ring wax was too thin in height and not thick enough to create a good seal around the bottom opening of the toilet potty. So i went to a home hardware store and bought another rubber real instead and placed it on top of the old toilet wax ring. How i came to know that the wax ring was not creating a good seal was that the wax ring looked like new and not compressed onto the bottom opening. Then i installed back the toilet potty on top of the new toilet ring. After that i tried flushing the toilet potty and have someone down at the garage tell me through the phone, if there was any water leak still. Turned out the fix was perfect and there were no more leak going into the garage ceiling. So i saved myself from having to pay for plumber to come and fix this issue. Sometimes the fix may not be as simple as the one i mentioned in my case. If it is something to do with the drain pipe or water supply source, then you might need a professional plumber to repair the plumbing problem. If you try to do anything out of your knowledge or capability your house could be flooded and you would have wished that you called a plumber to fix the problem.