Water leaking when shower is turned on

need some advice on how to look for the source of a leak around our bathtub. We have a tub/shower with surround walls. There is a small section of drywall on either side of the bathtub and the drywall on the same side as the shower head and nozzles has water damage. We at first thought it was condensation causing it and replaced the drywall and caulk. Over time we have noticed water damage again. Today when we looked at the wall it appears that there is a slow trickle of water coming out of the area where the surround and bathtub meet. It only happens when we turn the shower on. It doesn’t happen if we are filling up the tub. Any thoughts or advice on wear to look for a leak or what could be causing this. The drywall is only damaged from the point where the water is coming out and below. Thank you!