As commented by Ozzie1910, you should have record of your device serial number. Unless the device has already been removed from your AppleID account or device list, you will be able to check the Serial Number.
For iPad and iPhone, you can find your device list in settings:
Settings > [Your Name / AppleID] - scroll down to find the device of interest; tap the see the device details
You can also find device details simply by signing-in to your AppleID account from any computer using a web-browser:
https://appleid.apple.com
Having signed-in, open the Devices tab. Here you will find an entry for every device associated with your AppleID; tap the related entry to see full technical details of the device - including Serial Numbers and IMEI/MEID of Cellular devices.
Trade In
If shipping your iPad to Apple for trade-in, your iPad will be inspected and evaluated upon receipt by Apple's trade-in partner. If the declared condition of the device differs from that declared on-line, or a fault is found, this can significantly reduce or completely eliminate any residual trade-in value.
Screen defects of any kind can significantly impact the assessed trade-in value.
Many end-users prefer to complete the trade-in process at an Apple Store - and this (a) eliminates any uncertainty as to value and (b) provides opportunity to challenge the assessment. In-store, your iPad will be assessed - and a value offered; you can either accept the offer, or leave with your iPad in your possession. If you accept the offer, you will have the choice of using the value directly against an immediate purchase of a device of your choice - or take the trade-in value as a gift card to be used against a future purchase.
Anecdotal comments (from reports appearing within this Support Community) suggest that when “inspected” by technicians for warranty claims, displays exhibiting minor screen defects are generally considered to be fault-free - until such time as an inspection relates to a possible trade-in, at which point an affected screen might be assessed as damaged or defective.
