If your garbage disposal is leaking, the question of whether it needs to be replaced depends on where it is leaking. You will need to turn on the water and look under the sink. The leak will usually be coming from one of several places.
If the leak is coming from the collar area, the fix may be as easy as tightening the collar where the disposal meets the underside of the sink. There are screws that pull the sink flange down to squeeze out the plumbers putty and prevent leaks. There is a mounting ring that you twist to lock on the disposal. Check this area.
The drain areas are a common leak area as well. There are two outlets at the side of the disposal. The larger bottom outlet is the drain connection that the drain piping attaches to and runs to the wall piping. The smaller inlet on top of it is for the dishwasher air gap to connect to. This is held on with a hose clamp. If you have a leak here, either tighten the clamp, or you may have a tear in the hose.
Finally, if you have water dripping from the bottom of the unit and there are no obvious signs from a connection, you likely need a new disposal.