How to know which Apple ID is used to lock iPad if the display is dead? I bought an iPad mini (Wifi only) and gave it to my nephew as a gift. 
He switched on the device and set it up with his apple ID. He did not set up (Find My iPad/iPhone) feature.

After a month of usage, he noticed the device is heating up when connected to the charger. Suddenly, the device display is off, and can not be charged.

When connected to a USB port on a mac, or PC, it does not get detected.

I tried to hold Power button+Home Button for 10 seconds then releasing the Home button, but it did not work.

I tried to put the device in Recovery Mode, DFU mode, but nothing worked. The device does not get detected by Windows or Mac.

I toke the device to the service center I bought it from (Not an Apple Store, we don't have Apple Stores in Saudi Arabia). 

At the service center, they have a policy not to service any iOS device if it is locked using Find My iPhone feature from iCloud.

I told them that the feature was never turned on. They checked http://icloud.com/activationlock and entered the serial number, and found out that the device [Activation Lock : ON ]. So they asked me to login to icloud using the apple ID that was used to activate the lock and remove the device from [My Devices] in iCloud.

I logged in to icloud.com using my nephew's apple ID, but could not see the iPad in the list of devices.

I thought maybe the activation lock was done using another apple ID, so I tried mine, my brother's, all of the apple IDs I tried, none of them had this iPad listed among the devices in iCloud.

Now, I can not get service for the device because they think I stole the iPad.  Although I have an invoice that proves the purchase. 

I know that revealing the apple ID might be considered a breach of security, but if I could just get the first and last letter of the email of Apple ID used to lock the device, then we can reduce our efforts in this hassle.

My question is: how do I know the apple ID that is used to lock the iPad knowing only the serial number of the device?
Unfortunately, it is likely that your nephew temporarily used someone else's Apple ID in order to gain access to purchases that the other user had made.  My son did this on his device, against my rules.  If this happened, then that other person may have initiated the lock.  Since you have an invoice, proving ownership and the device is only a month old, I would encourage you to use the 1-year AppleCare that came with your device.  Here is the AppleCare contact page for Saudi Arabia.

Please let me know if this information is helpful for you.  Did you by chance register your device with Apple, when you made the purchase?  If so, then Apple will have a record of that action, as well.

Cheers,
Byron