Hot water temperatuer without a thermometer?

Looking for some help with a project. Im looking for a reliable way to check the temperature on a 2 "L" copper hot water feed to a dishtank. Being as this is a restaurant, shuting down the water to add a thermometer well is a non starter for the owner. On a past project i had a boiler rep use some sort of surface probe and would ideally like something similar but i have no idea what it was even called.

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