Is there any way of getting the serial number of my Seestar S50 from one of the FITS files I copied from the device onto my computer?
Are you not able to just look at the label on the bottom of the unit?
In PI you can possibly check the FITS Header information, but I don't recall a device's S/N being listed there.
No, you can check it through the Seestar APP, the name of the Seestar WiFi can also be seen, or you can just look at the bottom of the device.