Samadhi. This word has a big and a small meaning, according to what I understand about it. In the big meaning it includes the last three factors of the Noble Eightfold Path, which is often translated unfortunately into concentration. In the dividing of the Noble Eightfold Path into three: wisdom, morality and concentration, we prefer the phrase "Mental Development", rather than concentration, because in this big meaning of Samadhi there is Right Effort, Right Mindfulness and Right Concentration. So, it depends on what you're talking about when you use the word Samadhi, because it has this big meaning and also a specialized meaning for the eighth factor of the Noble Eightfold Path, that is Right Concentration by itself. So in the big meaning, we translate Samadhi as Mental Development.
"How does one know when one has reached it?" I think this person is talking about the small meaning of Samadhi, concentration states. But for the big meaning, how does one know when one has mental development, when one is making effort to prevent, let go, develop and maintain, and developing more mindfulness and concentration. As far as the concentration is concerned, how does one know when one has reached Samadhi, we don't usually talk too much about that. It's more about how does one use Samadhi. We're more interested in how one uses concentration, whatever level it is, because reaching it and gaining it is not the object of the practice, it's using it as a means of calming the mind. Any level of concentration that brings about tranquility is useful. It's not so important to define it, but certainly when one comes out of it, it's useful to analyze it. Analyze it as a state of mind that is impermanent, and because of impermanence it's not totally satisfactory and it can't be called "mine." This is what we prefer to talk about, that for any stage of concentration or tranquility that one has, to use it. And if one gets into a state of concentration where there's an experience, or whatever it is, when one comes out of it, to contemplate its impermanence, its unsatisfactoriness and its impersonal nature.