This is going to seem like a silly post and I won't argue that. BUT, sometimes the most obvious things need to be stated.
If you go to a water cooler and there are two taps in front of you and no markings telling you which is hot and which is cold, here's how you know.
Red is hot and Blue is cold. Think about it. Red is associated with hot things like fire. Blue is associated with cold things like snow and ice.
Here's another hint, typically the left tap is hot and the right tap is cold.
And there you have it. 😀
You Ought to Know: Red is Hot and Blue is Cold