How do I know a user is really the one updating themselves?

In FrontDesk, there are no guaranteed ways to prevent user's from swiping other users In or Out.  You must trust your employees to only edit themselves on the Board.

If you need a more secure solution that only allows a user to update themselves, check out FrontDesk's sister product: TimeClock.  TimeClock uses encrypted QR codes that you generate, print, and give to employees.  Employees use these barcodes to be checked In or Out by scanning at the TimeClock station where you would regularly have your FrontDesk station.