how can bigamy is proved

(i) Photographs of second marriage.

(ii)  Proof of parents name in child school records.

(iii) Second marriage registration  certifigate in case done.

Do you have the evidence of solemnisation of marriage?

Police and private detective may be able to help you.

Evidence is not required to file FIR.

Police can/may find evidence. 

You may take help of elders of your family, competent and experienced well wishers, seasoned PIP’s, helpgroups, community leaders, NGO’s, experienced colleagues, associations, religious scholars/leaders, influnecial persons etc and/or  find a very able LOCAL counsel/Law firm specializing in concerned filed of law e.g; Family/Criminal/Civil matters as in your case, and well versed with LOCAL applicable rules, precedence, latest judgments etc …. and worth his/her salt, can advise you after examining all case related docs, inputs,  evidences on record.

One should not fall for IT’s and entities loitering at online portals to allure unsuspecting querists. There are many threads on such instances at LCI also.

Online discussions are not substitute to in person discussions with a very able counsel of unshakable repute and integrity specializing in concerned field of law.

There are such very able counsels at each location.

Check for such counsels at LOCAL e.g; Family/Civil courts, HC, SC …

Show all docs and obtain proper written legal opinion.

You can also try for FREE legal Aid from Legal aid center (DLSA) that is usually within LOCAL courts complex..preferably from a very able counsel specializing in Family/Criminal/Civil/Civil matters.

If you are not satisfied with current counsel then engage another very able LOCAL counsel to defend your interest.