Yoga Sutras 3.12
Patanjali, explained by Swami Vivekananda
शान्तोदितौ तुल्यप्रत्ययौ चित्तस्यैकाग्रता-परिणामः ॥
śāntoditau tulyapratyayau cittasyaikāgratā-pariṇāmaḥ
The one-pointedness of the Chitta is when the impression that is past and that which is present are similar.
In plain words: The mind is one-pointed when each thought is the same as the one before it. You can tell, because you lose track of time, like when you are lost in a good book.
Swami Vivekananda explains
How are we to know that the mind has become concentrated? Because the idea of time will vanish. The more time passes unnoticed the more concentrated we are. In common life we see that when we are interested in a book we do not note the time at all; and when we leave the book, we are often surprised to find how many hours have passed. All time will have the tendency to come and stand in the one present. So the definition is given: When the past and present come and stand in one, the mind is said to be concentrated.