"Αυτή" is "she" singular a female. "Αυτοί" is plural male or female. If it's written you see it.
But if you hear it in a normal conversation you'll understand by the context.
But if you had this in a listening exercise there is no way to know. So, please tell us if you had it on a listening and we'll report it for removal as it's confusing. Thank you.