You may need to know everything about a phone before you buy it. The appearance matters, of course but one may not understand what the phone offers in terms of functionalities.
FROM GSM ARENA
Go to www.gsmarena.com, input the phone e.g Samsung Galaxy Note FE and the phone specifications will be displayed.
The following screenshot will display EVERYTHING about the phone.
FROM THE PHONE ITSELF
If you have the phone with you, navigate to Settings > about phone.
It will display every information about the phone, including serial number.