First steps to isolate leak?
Cutting away the drywall is one possible first step, with other possibilities being to see what is above the ceiling, depending on ability to access.  That section of drywall will definitely need to be removed anyway.  (both to repair, and to check for mold).  Finding the source can be difficult because the source of the water might not be directly above.  Water can run along various boards to find a low place to settle.  It could start at some point on an outside wall and go a fair distance horizontal.