The easiest seems like who is paying the "tip". If it's your employer, it's taxable compensation. If it's a customer, than it could be a tip.

Then there would probably be a bunch of sub-bullet points defining whether a tip is taxable depending on whether you have a direct relationship with the customer tipping you, whether the "tip" is mandatory, or at the discretion of the customer, are you in X roles (e.g. barista) within Y industries (e.g. food services).