Step 1
Flush the toilet and smell the air in the room surrounding the toilet, immediately after the toilet flushes. Try to notice the a sewer smell. Examine the floor around the toilet for signs of water leaking if you notice the sewer smell. Water indicates a compromised water seal instead of a cracked drain pipe.
Step 2
Enter the crawl space directly underneath the toilet, and locate the toilet's drain pipe. Examine the pipe for any leaks, as someone in the bathroom flushes the toilet.
Step 3
Look for a water spot on the ceiling directly below a toilet on the second floor or higher that you suspect has a cracked drain pipe.
Step 4
Replace the wax ring on a second-story, or higher, toilet. Look at the ceiling directly below the toilet in the days afterward, and see if the water spot continues or dries up. If the leak and the water spot continues, the leak is in the toilet's drain pipe, instead of possibly being a faulty wax ring.
Step 5
Climb a ladder, and remove ceiling tiles directly below the toilet on the next floor above it, to expose the drain pipe, if possible. Or, place plastic on the floor, and cut a hole in the ceiling directly below the toilet above, using a tool such as a keyhole saw, to expose the toilet drain pipe.