Water Damage
How do you know if your phone has water damage
iPhone 11, iOS 15
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Usually there are signs of it. Other than that, it requires disassembly. An Apple technician or Apple Authorized Service Provider would be able to tell by looking inside. There's a liquid contact indicator inside the SIM card slot. It wouldn't necessarily show liquid damage but if that (or any of the LCIs inside) show liquid contact, it's going to be ineligible for service from Apple or an AASP.
User accessible LCI locations are in this article:
Water and other liquid damage to iPhone or iPod isn't covered by warranty - Apple Support