How do you retain a phenotypes genetics when you get a good cross?

Ummm depends where the seeds came from.. what pollinated it? Did you pollinate it purposely or were the seeds accidentally made from random pollination?? Best scenario is to take clones man.. breed with those. If you didn’t purposely pollinate, you don’t know half it’s genetics, and are back at square one basically having to pop lots of seeds and do a Pheno hunt.