How do you know when you have to credit the icon designer?

If you have to credit the icon designer, it will be specified in the license agreement (where it says License: ...). As a rule of thumb, licenses that have the word "attribution" in their names (mostly Creative Commons licenses) require crediting the icon designer. For other license types, crediting a designer is optional.

How to properly credit the icon designer?

You can do this by including their name and a link to their website or to their Iconfinder page. Here are some good examples of How to credit the icon designer.

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