Open Facebook and go to the profile
Check the profile URL in the browser address bar
Look for the part after facebook.com/
If the URL shows a custom name, that is the username
If the URL shows numbers, the account may not have a custom username
On the Facebook app, open the profile and tap the three dots
Select Copy link to profile
Paste the link and check the username in the URL
Use the Facebook search bar to find the profile if needed
Ask the person directly for their Facebook username
Check the About section of the profile if it is visible