Warning! Most automotive oils and fluids are poisonous. Wash them off your skin and change contaminated clothing immediately.
The smell of the leak can tell you what exactly is leaking. Some fluids have a specific color. It is useful to wash the car thoroughly and put clean paper under it overnight. to determine the location of the leak. Note that some leaks can only occur when the engine is running.
Oil on the pan
Engine oil may leak from under the drain plug.
Oil from the filter
... or from under the filter.
Transmission oil
Transmission oil may leak from the seals that seal the inner bearings of the drive shafts.
Antifreeze
Leaking antifreeze often leaves behind crystalline deposits like these.
Brake fluid
Leaks on wheels are almost always brake fluid.
Power steering fluid
Fluid may leak from the pipe connections on the steering rack housing.