Determining which API key is supposed to be a secret isn't usually an issue, cause typically site providing you with the secret key clearly states the fact that

Determining which API key is supposed to be a secret isn't usually an issue, cause typically site providing you with the secret key clearly states the fact that it's supposed to be a secret. For example this is what OpenAI says about their keys directly on the page where you generate the key: "Your secret API keys are listed below. Do not share your API key with others, or expose it in the browser or other client-side code."