How do I know if a device I bought at a thrift store was stolen?

If you have knowingly purchased a pre-used device from a thrift store - and the iPad is neither disabled or activation locked - why would you now worry that the device is a stolen device? Short of returning it for a refund, there's nothing that you can do to change its status.

Other than the iPad being locked with unknown credentials, this being an indication that the device was not correctly prepared by its owner for reuse by a differently user, you have no accessible method to identify the reason for it being locked - whether stolen or otherwise. Apple have no means by which to advise you of the ownership status of the iPad; even if they did have the means to so, they would not be able to share this information with you (or anyone else) without transgressing privacy laws and legislation with which they must comply in many regions/countries through the world.

Should you choose to do so, you can yourself perform a full reset of the device. If there are any latent issues with an undetected Activation Lock, the reset will trigger it - and you may lose use of the iPad as a consequence:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Here is what Apple suggests when purchasing a previously used device:

Check for Activation Lock before you buy a device from someone else

Before you buy an iPhone, iPad, or iPod touch, from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account. Follow these steps to check if the device is ready to use: 

1. Turn on and unlock the device. 
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, contact them as soon as possible and ask them to use a browser to remove the device from their account. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.

You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.