iPhone xs max

Enter your iPhone serial number in Apple's webpage Check Your Service and Support Coverage. A genuine device serial number will give you a result.

You can find the serial number at Settings > General > About > Serial Number

Touch and hold on the number and you then can copy it to clipboard for passing in the webpage I have linked above.

beeaware wrote:

must i write serial here?

No, do not post the serial number here. Instead, enter it at the link provided by David above.