If your transmission fluid is leaking you should notice a slick red puddle under your car.
The leak could be coming from the transmission pan gasket, or could be a front or rear transmission seal leaking.
If you notice a red puddle under your car you should check your transmission fluid level and see if the fluid level has dropped.
If you add some more fluid and then later it drops again then your transmission is leaking fluid and you need to have it checked out.
Sometimes it will require a shop pulling the transmission to replace a front and rear seal if it's a front wheel drive car.
On a truck you can usually get to the rear transmission seal by removing the drive shaft.
But for the front seal the transmission will have to be removed.