Check your TSA PreCheck approval letter or email
Look on your Known Traveler Number (KTN) card if you received one
Log in to your Trusted Traveler Program account
Check your airline frequent flyer profile if you saved it there
Review past flight bookings for the KTN field
Contact TSA Customer Service if you cannot find it
Contact the enrollment center where you applied
If you mean your Redress Number, check your DHS TRIP account or approval documents