The critics of the birth-based caste system and those who support the behavior-based caste system, cite these verses from the Bhagavad Gita in support of their theory:
Bg 18.41 — Brāhmaṇas, kṣatriyas, vaiśyas and śūdras are distinguished by the qualities born of their own natures in accordance with the material modes, O chastiser of the enemy.
Bg 18.42 — Peacefulness, self-control, austerity, purity, tolerance, honesty, knowledge, wisdom and religiousness – these are the natural qualities by which the brāhmaṇas work.
Bg 18.43 — Heroism, power, determination, resourcefulness, courage in battle, generosity and leadership are the natural qualities of work for the kṣatriyas.
Bg 18.44 — Farming, cow protection and business are the natural work for the vaiśyas, and for the śūdras there are labor and service to others.
Now at first glance, one might think that someone's outward behavior as listed above determines their predominant guna.
But the problem with this theory is that people's gunas and behaviors constantly change, people can make their behavior change through practice, and people have a mish-mash of the qualities listed above that aren't cleanly divided along those lines.
So, one must resort to determining the person's actual guna composition through some other means.
How do the supporters of behavior-based caste theory go about determing someone's guna composition? Through siddhis?