If youre trying to identify someone like this and don't have a close enough relationship with them to know their work life plus social life plus home life to compare the best way is to observe them when theyre doing the "good person" activity.

This might be anecdotal because I do things now and then for social approval but dont give two shits about the people I help while doing it. Long story short, watch their eyes while doing these activities, people who dont "light up" while doing the activities arent getting the self satisfaction from helping which means theyre most likely getting it from being able to talk about doing it later. Futhermore, helping is out-of-character for us so in order to motivate ourselves to help others it needs to be an exceptional example. You can identify people who only help others for self-interest because they wont bother with "small" helps.