How does one validate that ad traffic is coming from a real user of the app and not a bot?

Question

I have an app that has been on the Play Store for many years and using MoPub for many years. The app has dozens of millions of downloads and hundreds of thousands of daily active users.

All of the sudden MoPub claims I have device ID stuffing. I'm pretty sure I'm not doing that since I'm not doing anything different or odd or trying to cheat the system in any way.

They sent me a page that has some "helpful" hints and one of them says Do you have a method of validating that impressions are served to legitimate human users?

So how would I go about validating that on my own app?

Edit: just to be clear, I want to know how to validate that the ads are being loaded and clicked on by a human.