The realisation of Dhamma is like sunrise. You start out in pitch darkness. You see nothing. You are clueless. Your ignorance makes you blind.
When you start to practise along the correct path, and understanding grows, the darkness that envelops your mind will begin to fade. Dhamma concepts begin to make some sense. You know you may not have gotten it but you know enough to be able to tell what is or is not Dhamma. Just as you can make out the forms and shapes enough to avoid the dangers and troubles, when dawn’s first light start to cast faint, weak glow upon the land and dissipate the shrouding darkness.
At some point, as your understanding of Dhamma matures, every concept that once perplexed you will start to make sense. Their place in the whole teaching, their role, their purpose, their meaning, will all become clear. The process of realisation is literally like the dawn of a new day: the whole landscape of reality is now bathe in the light, clear, obvious and beautiful.
Just as you cannot exactly pinpoint when night turns to dawn, you may not be completely sure when Dhamma has truly taken root in an irreversible way. But you will know when it is day break. That the night is past and the sun has arisen.