law: Indian Evidence Act, 1872 
  50. Opinion on relationship, when relevant 
  When the Court has to
  
  form an opinion as to the relationship of one person to another, the opinion
  
  expressed by conduct, as to the existence of such relationship, of any person
  
  who, as a member of the family or otherwise, has special means of knowledge on
  
  the subject, is a relevant fact:
  Provided that such
  
  opinion shall not be sufficient to prove a marriage in proceedings under the
  
  India Divorce Act, 1869 (4 of 1869) or in prosecutions under sections 494, 495,
  
  497 or 498 of the Indian Penal Code, (45 of 1960)
  Illustrations
  (a) The question is,
  
  whether A and B, were married.
  The fact that they
  
  were usually received and treated by their friends as husband and wife, is
  
  relevant.
  (b) The question is,
  
  whether A was the legitimate son of B. The fact that A was always treated as
  
  such by members of the family, is relevant.
  Illustration
  An expert may give an
  
  account of experiments performed by him for the purpose of forming his opinion.