It may turn dark or black with use. It commonly lubricates manual transmissions, axles, and differentials. It is also used in the transfer cases of some import 4x4s. Small drips ordinarily aren't cause for alarm, but check for signs of water in the axles and differentials. Water is a certified gear killer.
Gasoline is a thin, volatile liquid with a distinctive odor.
Diesel is a light oil, usually dyed red. It also has a distinct strong odor. The leak could occur front or back, depending on whether it originated from the tank, the fuel lines, or fuel filter. Given the extreme fire hazard, any fuel leak is a major problem, and should be corrected immediately.