How does one know if one is destined to attain Nibbana? How does one know if one is destined to attain Nibbana? (In future lives)
Traditionally, one hears it from one's teacher (Buddhist mentor). For example, my Zen master once said, pointing at me, "One day this guy will get everything. He will be Bill Gates". This is known as "the prediction".

Even without that, the teachings say, if you are committed to figure it out, however slowly, you will get there sooner or later.

This abstract idea that Nibbana is actually within reach gets concrete and personal only at pretty advanced stages. Basically, once you clearly see the mechanism of conflict/suffering and how it all works, and not just see but get some grasp of mastering unconditional "rightness", that's when you know you will get there with time.