How do I determine cause of leak in fiberglass shower stall?

1 piece or 3 piece? In a 1 piece it is almost always the drain, but it could be in the plumbing inside the wall. In a 3 piece it could be all of the above and leaks in the wall sections.

Where exactly does the water appear ? Is it continuous leak or only when the shower is in use ? Do you have shower curtain or glass door at the entrance to the shower ?

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Milo