Answer:
When you think about it you can easily know by someone's body language they look uncomfortable when with a group the would not like the situation or they feel like there being outcasted if the decline invitations with a group or etc it's clear there don't feel wanted and this can be cause cause the may be invited but can be excluded but you can also see ex how when there mood changes when around the person or people not treating them right but then there can be marks if not being treated right is domestic but if they can also tell people how they feel there being treated or as said mood change when ever you see them or the seem more down alot or red flags for you to see when your not being treated right disrespect aggression a bad attitude towards you for no reason or something doesn't feel right