Even easier, pull the dip stick on both the tranny and motor. Smell them both then smell your mystery oil. If it is "Black" chances are it is motor oil, tranny fluid will have a bit of a redish tint to it, even if it is burnt.
If it doesn't smell like either one of them then you need to check your front & rear axles for leaks.
When is the last time you changed your motor oil? If it is black and smells burnt, then you need to change it. Should be changed every 3K miles.
If you it is motor oil then you might want to try a "High Mileage" motor oil, that will help slow down your leak.