Look for it. Black mold can be hiding under the sink or around the toilet. Check in the basement, where it can start and move up into the rest of the house. It is a black mold that will take over entire walls and ceiling tiles. It usually can be easily found in moist areas.
Detect it in places where you can not see it by the smell. Mold has a musty smell, like old things that have spent years in the attic. Black mold can also have an earthy, fresh dirt smell. Since black mold can grow inside your walls smelling it is the only way to find it without doing a mold test.
Do a mold test. Though you smell it you want to make sure that it is mold before you go tearing down any walls. Hardware stores sell mold detection kit that can find mold in the air or on surfaces. So even if you see something you think is black mold under the toilet you can test it to make sure it is not just dirt.