How to authenticate rapidAPI requests

Question

I have a website where I have a sports API available, every user has a limited number of requests and so when they make a request to the site, my node/express backend checks for an api_key to authenticate the request.

I've recently listed the API on rapidAPI, and when a user makes a request from rapidAPI, they're supposed to use their rapidAPI api key that is given to them by rapidAPI. I know that rapidAPI will check if the api_key is valid before sending the request to my API, but how do I know whether or not to check for the api_key when people are making requests from my website URL.

I would expect the express function to do something like this...

router.get('/v1/:sport/odds', async (req, res) => {
    // if the request came from rapidAPI and has been authenticated by rapidAPI, continue...

    // if the request didn't come from rapidAPI, check for an api_key in req.query and then continue if api_key is valid
})