How to tell whether my 20W power adapter is original or fake?

Hello! I purchased a 20W power adapter for my device from a local store, and after some time of usage it looks like a fake charger to me. The seller, however, insists that the device is original, and we're having a dispute now.

Is there any reliable way to know if I bought the original charger or a fake one? If so, how can I check?

I attach the pictures of dissasembled device, if it can help in any way.

Thanks in advance for the answers!

[Image Edited by Moderator to Remove Serial Number]