Could anyone tell me how I could know the following?

await page.setRequestInterception(true);
page.on('request', (req) => {
   if (req.isInterceptResolutionHandled()) {
      return;
   }
   //---------------------------------------------------------------------
   //HOW TO KNOW IF CURRENT "req" IS DUE TO AN "<iframe>"'s DOCUMENT LOADED?
   //---------------------------------------------------------------------
});
await page.goto('https://web-site.com/web-page-containing-iframes');

I have tried without success executing the following fragment (just replacing the comment):

let frame = page.frames().find(f => f.url() === req.url());
if (typeof frame !== 'undefined') {
   console.log('EUREKA!');
}

Obviously it doesn’t work because the new URL hasn’t been loaded into the iframe.