How can I ensure that my defective device has really been replaced?

I bought about a month ago a notebook, this had unfortunately damaged a battery and has constantly discharged. That's why we contacted the seller and sent it in with the agreement that a new device will be sent to me. After five days I had the new device with me while I unfortunately had to realize that there were already numerous fingerprints on the screen and it was trying to clean with a damp cloth, in addition, all the images (which were actually deleted when the reset of the old device) , WLAN access, wallpapers, etc. All still available on the notebook.

For this reason, we have the suspicion that the notebook was never replaced, but the seller himself says that this device has been replaced and that he can't really explain the fingerprints, etc. Now my question would be how can I testify that the device is definitely not is new?

The device has been repaired - which ultimately the manufacturer does; is solely at his discretion; after damage assessment.

the manufacturer gives the guarantee - the seller the warranty.

Do you have the SN no. Recorded on the old device and compared with the new device?

Compare serial number of the purchase contract with that of the alleged new device.

The manufacturer said in our phone call, however, that it was surfaced

Then the manufacturer is your contact person, office and workshop have decided contrary