It sounds as though you received pre-check from TSA as the result of your UA status back when that's how it was done. You don't have a KTN. You can obtain pre-check stand-alone either through Global Entry ($100 for 5 years) or TSA's own pre-check program ($85 for 5 years). Either program will generate a KTN which you can insert into your DL and AA profiles as well.

TSA has announced that those using a KTN will experience a higher % of pre-check selection than those relying on the older free program. If you do any international travel at all, GE is well worth the extra $15 because it vastly expedites CBP processing as well.