Is that possible to check an iPad's total power-on lifetime since its manufacture? I just bought a brand new 2019 iPad Air. But I am afraid it's not a brand new one. I know I could check the model or serial number, but I don't really trust that approach.

I think it's best to check the power-on lifetime if there is a hidden feature in Settings somewhere (or enter some magic numbers in the dial-up panel like what Android devices do).

Thus is it possible to check iPad's total power-on lifetime since its manufacture?
It is not generally possible to determine the total power-on lifetime of an iOS device.

To determine how much the device is used, a better indicator is to determine the battery charge cycle count. A single charge cycle count is equivalent to one complete discharge or the device's battery. For a new device, the cycle count value is a low value between 0-2.

To determine the battery charge cycle count, you can use an app such as coconutBattery.



Additionally, if you are running the current latest version of iOS 12, you can determine is still covered under limited warranty. On your iPad, go to Settings app → About and look for an item named Limited Warranty. It will show the warranty expiry data. The devices whose warranty has expired, this item is not shown altogether.



The last mentioned approach uses the device serial number and fetches the warranty expiry date from Apple's servers. Since the limited warranty for iOS devices bought from Apple is for 1 year from the date of purchase, I think it's a good measure that can be used to determine the genuinity of the device. 

These two approaches should help you with confidently getting answer for your concerns.