@Masafumi99 What prompted my question was words like 人種、人制、軍人。I think I heard something about "じん" being the reading when the word is associated with a person's identity? But I can't remember exactly how it was phrased.
@PastaEnthusiast Just as with other Kanji with multiple readings, it’s 音読み for compounds (in this case ニン) and 訓読み when alone or as part of a verb/adjective with おくりがな (for example, 楽 is read たの in 楽しい, an adjective, but ガク in 音楽, a compound).
So, 良い人 is not a compound, so it’s ひと。
老人 is a compound, so it’s read ジン (which is another version of ニン).
But there are exceptions. Especially for very broad, often used kanji like 人. For example, 人目 is pronounced ひとめ (people’s eyes), even though it seems like a compound. 二人 is pronounced ふたり, which doesn’t seem related to either of the readings. So with such a fundamental kanji, you should focus on learning many compounds, rather than guessing based on the possible readings.