You have to use a third-party solution. There are several on the iPad App Store. Search the store for 'battery health."
However, I cannot recommend battery service on any iPad Air 2 as they can run no higher than iPadOS 15, and version 18 will be out soon. A number of apps are now requiring iPadOS 16, so you'd be spending money on a device already on the brink of "unsupported"
If the battery is otherwise healthy, it will continue to serve at under 80% health. Real world example: We have an iPad 2 bought in 2012 that is still in daily use for old game with the original battery. Its health is about 70% and cycles are nearly 1300. Yes, runtime per charge is down from new, yet acceptable for the tasks it does.
—If your battery is showing tolerable runtimes before needing recharging, I would continue using this iPad until it no longer supports your needs.
—On the other hand, if the battery is to the point where you cannot use the iPad at all, like the charge falling from 100% to nothing in minutes, that battery has failed. I recommend consider a new iPad that will be relevant for five of more years. Apple will charge US$119 for a battery servie in that model, money better saved for a new iPad.