How to check if Admob ad is shown and is watched?

Can anyone help me with AdMob Free. I had call my AdMob ads, and they are shown every time. Now, I need to check if Rewarded Ads is shown and watched to the end. I use this code:
document.addEventListener(‘’, (result) => {
some code to reward the user
This code is working OK, but I have problem with that if Rewarded ad is shown at all. Because when I publish my App on playstore, in first time there is no ads, but they are shown later.
So my question is how to check if Rewarded ad can be shown, so then I can call a code above?