The Fan Tray serial numbers are listed in the outputs of show chassis hardware.
 

To check the Serial number of a Fan Tray using the CLI, you can use the command >show chassis hardware.
 

Also be advised across Juniper devices there are various devices in which the show chassis hardware don't reflect the Fan tray serial numbers.
 

In those case we have to manually check the Fan tray serial numbers.

1. Physically Remove the Fan Module: You need to remove the fan module from the switch chassis to access the serial number ID label. Ensure you have the required parts and tools available for this task.
2. Locate the Serial Number ID Label: The serial number ID label is typically located on the top of the fan module.