Final answer:
Underwater coastal plains can be identified through the analysis of evaporite deposits, caves formed by acidic brines, and the presence of fossils.
Step-by-step explanation:
The presence of underwater coastal plains can be determined through various geological evidence. For example, the analysis of evaporite deposits and caves formed by acidic brines can indicate that the coastal plains were once underwater. Additionally, the discovery of fossilized remains of marine organisms, such as silica-rich sponges, in the coastal plains provides further evidence of their underwater history.