Another pretty big clue will be if your roof is completely rusted over.
I have a leak in my car roof.
If you spot any rust near the roof sand it clean it with rubbing alcohol and maybe even use some touch up paint on top.
If there is a leak and the inside of the car smells like coolant a sickeningly sweet smell when it gets warm the heater core may have sprung a leak.
If you open it all the way you should be able to see them.
However if you can see any bubbling or cracks in the paint along the sides that s where your leak is.
Check to see whether the liquid collecting in the car is antifreeze or water don t taste it.
They re equipped with drainage tubes that carry excess water from the roof down and then off your vehicle.
Once you have determined that the leak is caused by a roof related issue measure the location of the leak in the living space from two fixed points.
In fact the roof is built to leak albeit in a controlled fashion.