It seems that you don’t know with certainty whether this is a pipe leak or a roof leak, so that’s the first thing to find out. If it only drips when it rains, then it’s a good bet that it’s a roof leak, of course (though it could still be a roof drain if you have a flat roof with a built-up parapet, which is common).
If it’s a roof leak, then there’s almost no telling where the leak originates. Those things are notorious for coming in at any odd crack in the covering, a piece of damaged or unseated or improperly lapped flashing, or a piece of tar that held some flashing in place coming loose… and then that little bit of water can wander the width of the roof as it flows along a ceiling joist or roof beam until it finds the ‘right’ place to drip.
If it’s a pipe leak then it should be easier to find, since all you have to do (easy to say “all you have to do”, but harder to effect) is trace the various pipes in the ceiling to find which joint is leaking. But that usually is easier to do.
Assuming a roof leak, I’d recommend that you buy a 5-gallon bucket of roofing tar and a water hose and go on the roof during a decent spell of great dry weather. Thoroughly wet various parts of the roof and do the best you can at keeping the water contained in the smallish areas that you want to test. Check every tarred seam, piece of flashing, roof penetration, vent and drain that you can. Start with the area right over the leak (you may be incredibly lucky to find it there, but don’t count on it) and work outward slowly. If you end up doing the whole roof in one morning, for example, then you’ll still not know where the leak originates if it’s a small one and it migrates after it comes through the roof.
If you can isolate the “general” source of the leak then tar the hell out of that area, with particular attention to any place that shows a seam in the existing covering, a penetration, flashing, etc. And if you do have roof drains, make sure that they are unblocked and free-flowing, because water backing up in a drain can find a pipe seam and leak through the pipe, and it can also be dangerous to the structure to have undrained water backing up in a heavy rain.
Good luck.