I am not really one to talk since I don't have any practical experience in my own right, but speaking as a newb...

How do you know you're straight or gay if you're a virgin?
Some people know instinctively, while others have to experiment based on the assumptions they have of themselves.
Likewise I think some people are naturally going to have bdsm urges whether they realize that's what they are experiencing or not, while others may only stumble into this realm by accident, and then realize that they are interested. A married middle aged man with several children can suddenly realize that he's more interested in men than his wife.
I think we have to name ourselves when we are comfortable with what we are, because no one can name us Dom/sub when that can change as you learn and grow within your own experiences. And only you can fully realize when that label no longer applies.

I think in terms of declaring ourselves Dom/sub
We know when we discover our urges and acknowledge them.
We do when we go out of our way to pursue these urges, whether online or in reallife.
We are the instant that we are realize that we are comfortable with being and fully embrace that aspect of ourselves.