Serial Check
Check the shape of a factory serial before you take a listing at its word. Matching a known format is a start, not a verdict — it says nothing about warranty, activation lock, or whether a handset has been reported stolen. Those answers hang off the IMEI.
Where the serial is written
On the housing
Some older iPhones have it etched into the back panel, and Samsung prints it on a label under a removable cover. Either way the number is attached to the shell, and a shell can be replaced.
In the software
Open Settings → General → About on an iPhone or iPad, or Settings → About phone → Status information on a Galaxy. Buying second-hand, read both and compare them: when the screen and the shell disagree, the case has been changed and the two numbers describe different objects.
Apple serials
Recognises the 12-character format Apple used for years — a leading letter, then a mix of digits and capitals, of the kind that starts F2LGR. Not every Apple serial is written that way, and a match confirms the pattern only, never the unit it was printed on.
Samsung serials
Recognises the 11-character codes Samsung prints on the housing, on the label beneath a removable back, and on the box. Read it off the phone rather than the packaging: boxes and handsets get separated all the time, and a seller may be quoting one to describe the other.