How would the average person realise that the use of "we" meant others were involved in the decision and not a case of someone using the "royal we"
Because others end up doing what the person in question said "we'll do". Either they were part of the decision, or he is powerful enough to make them follow; the result is the same.
Of course it's powerful rethoric; the use of the plural itself serves to cement the group, not only in the eyes of its enemies but internally too.
