When should I check this checkbox ?
and If i'm about to open a rar , how can I see the authenticity ?
After open the rar archive with winrar press alt+i (extras -> get information). It shows stats about the archive and if available, the info about package creator.
From the options page are winRar:
From another website:
Only turn this option on if you want others to be able to check that you make this archive and nobody tampered with it.