Look at the rubber seal which will either be on the Tailgate or the lip of the Tailgate and see if it is still soft to touch, complete and shows no tears. Also look to make sure that there are no leaves or other kind of dirt that would stop rain water from flowing around the outside of the seal.
If the vehicle has a rear washer/wiper that stores its washer fluid in the tailgate area, some can leak if they are overfilled. Check the bottle to see if is damaged or if the hoses that run from it are cracked split or missing. Some have a hose that runs from the washer bottle at the front of the car, make sure that this is not damaged by watching for leaks when the rear washer is used.