The fact that I get one of these wrong is one of the most common reasons for why something gets sent back to Apprentice and quite often it feels like when I get it right, I just happened to guess right, felt like a coin-toss rather than me actually remembering it.
人 - If it’s a suffix, there is a pattern. This doesn’t include cases where it just happens to be the 2nd kanji in a regular 2-kanji compound. These are pretty simple… じん - nationalities / demonyms, にん number of people, or person who does an action.
If it’s not a suffix, there isn’t a pattern. The readings are based on when the word was imported from China. にん is older and じん is newer.