That can be a tough one unless you are willing to open the ceiling and see where the leak is comming from.
If you are not willing todo this at this time you will have to play detective and go through a process of elemination. First, most the time it is the grout and/or tile around the tub, check that first and repair or replace as needed and see if that help’s don’t forget to water proof around the sides of the tub and where the tub meets the floor. Second, if it rains and the spots get bigger you know what you will have fix.
There are more steps but try these.