Your question is valid.
But it needs thought, polishing...
A chart might show that the person tends to lean toward things that go Boom
Or is clinging, and thus prone to collecting Barbie dolls
If the whole-tone of the chart is conservative, cautious....you are not likely to find a race car driver or antarctic explorer.
Bruce Lee's chart had Mars as final dispositor, Mars the Warrior. There's his passion....Mars was in Scorpio (Purification). There's his passion. Other things in the chart continue to modify, to refine, to specify......until finally the astrologer's intuitive mind (which largely works on accumulated experience) is quite likely to "see" what the passion is, that is, the life purpose.
The word "passion" comes from the Middle Latin passio, which directly refers to the Passion of Jesus, and thus came to signify "sacrifice, suffering".....and then it came to be applied to the sexual and other passions. Therefore, a "passion" is, in a sense, "what you would sacrifice yourself to, would die for."
On dog on the trail of a ***** in heat is oblivious to all around him. There is only one thing that concerns him, and he would die for it. Passion.
The heat of anger: you see red and there is nothing in the world but the object of your anger and the anger itself. The passion consumes you.
The Bhagavad Gita says that if a man truly worships a god, and makes sacrifice to that god, then that god will reward him. This is passion. What consumes you, what you are willing to sacrifice yourself to and suffer for, that is your passion. It is the passion of love, of loving so much you give yourself to it, lose yourself in it.
And the horoscope does show this passion. But it won't be found in the way you were looking for it.
The "seer" must see the whole. Once you have taught yourself to do this, then you will be able to see the parts (whatever sort of detail you want) and understand them in relation to the whole.