There is no way to determine just what a pony has been through with one single glance, and it is an impossible task; even for detectives. The only ones who really understand anypony is the pony themselves, but that doesn't mean you can't try to. If given the opportunity to either shun sompeony or to accept them, always go for the second option, and never make a pony feel left out.

Constructive criticism is appreciated.