Hello, and welcome!
If you are referring to your TSA Precheck number (or KTN---Known Traveler Number), you can retrieve it by going to the following webpage and filling out the form. Even though it says 'status', it will provide you with your number, as long as your information matches what they have on file for you
You can also call Universal Enrollment Service at: ***-***-****
Yes, Delta is part of the TSA Precheck program, but please be aware that Precheck is a randomized selection system, which means even though you have it, you might not get it on every flight; this is for added security. It would be a good idea to tell Delta and any other airlines you fly frequently, to add your TSA Precheck (KTN) number to your profile, so they always have it.
I wish you a great flight and a wonderful Holiday Season!
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You can view more about Delta and TSA Precheck, HERE