How to check if iPad battery is original?

There may be different interpretations of the word "original" here. Do you mean the very first battery even in the iPad, or do you mean an Apple authorized battery = genuine?

"An Unknown Part  message will appear if the battery installation is incomplete or if the battery:

• Was replaced with a nongenuine battery

• Was already used or installed in another iPhone

• Isn't functioning as expected"

An explanation of that message is in the article 'About genuine iPhone batteries' --> About genuine iPhone batteries - Apple Support

I would not be concerned if the battery has been replaced. To the contrary, it would be nice to have an unused battery in the iPad. What may be concerning is if it was replaced with an Apple battery or a third party one. If third party, a warning message should appear.

If it was not replaced and the iPad has been used a lot, an original battery may show usage. You should realize that but also take into consideration that you are getting a used device and cannot expect it to be brand new. iPhone Battery and Performance - Apple Support

To see the feature go to: Settings > Battery > Battery Health

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." A degraded battery may display the notice: "Your battery’s health is significantly degraded. An Apple Authorized Service Provider can replace the battery to restore full performance and capacity." If it is not at or above 80% "normal", consider contacting Apple about a battery replacement. --> iPhone Battery Replacement - Apple Support

If they truly "refurbished" it then that should include battery replacement with an Apple battery. The information provided earlier in my post and others should enable you to determine the age of the battery and if it is an Apple one or not.

I can't find that battery history function on my iPhone SE running iOS 17.6.1. Maybe it's on more robust phone models. I don't see it on either iPad in the house either.

Because taking apart an iPad is such an involved matter, I would be suspicious of any refurb NOT bought directly from Apple. When Apple refurbs an iPad, they install a new battery and a new outer shell.

However, a couple of thoughts:

1) Do "Settings > General > About" and look at the model number:

If the model number starts with a "F" as in the example, it is a genuine Apple refurb. I doubt a third-party refurbisher knows how to reset the model number.

2) If you have a Mac computer, the third-party app Coconut Battery will read the battery's manufacture date:

In the example, note that the phone was made in July of 2021 and the battery in June 2021, That is typical for an ORIGINAL battery based on our Apple devices. If the battery manufacture date is AFTER the device's date, then the battery has surely been replaced.

Also, if any of those fields return a null value, that could indicate someone other than Apple have been inside.

I'm not sure if this applies to iPads but if the battery was replaced, "Parts and Service History" should appear in Settings > General > About > Battery (and you can see if the battery is a genuine Apple part or not). If nothing appears, either the battery wasn't changed or this data isn't available on an iPad. Perhaps someone else here can confirm about the latter possibility.

Regards.

If you had your iPad battery replaced by Apple or an Apple Authorized Retailer, the battery is most likely original.

-Zachyy ✌️

I think that is the way to find out if the battery was replaced, though I believe the feature only applies to certain iPad models.