Let me again tell you, to make the criminal cases relating to bigamy definitely requires the proof of both marriages, even if first or second or both marriages have been admitted by accused that will not be sufficient enough to hold him guilty under section 494,495 IPC.
Secondly proof like voter's card, ration card, official records, living together evidence by neighbors or DNA report of child born out of such union does not prove the marriage ceremony, for birth of child one doesn't have to marry, nor for living together or declaring oneself married to the other person, these will not prove the actual marriage ceremonies which court will like to see and come to conclusion of actual marriage between two persons.
When I state proof of both marriages I mean proof of marriage ceremonies in both cases, a child of 10 years will not even be aware what are the essential marriage ceremonies required for valid marriage and in this case how can you expect the child to state the same in the court, even are you yourself aware of the essential marriage ceremonies in the two marriages in question? Most of the people confuse between Saptapadi and Phera, rather most call it taking seven steps around the sacred fire which wrong, it is taking seven steps, each step representing one vow chanting by the priest along the sacred fire not around the fire like Pheras. Likewise there are number of essential ceremonies in Vedic or Sanatni Hindu marriage, essential ceremonies in Northern part of India may be different in Souther part of India for Hindus, likewise the Muslims, Parsis, Jews and Christians have their essential ceremonies for marriage.
Witnesses who understand these ceremonies and who have actual witnessed these ceremonies can stand as competent witnesses, when you take a witnesses to make as prosecution witness in bigamy criminal proceedings you need to completely tutor him otherwise defense counsel in cross examination will get truth out of the witness that he attended only the wedding reception but did not witness the essential marriage ceremonies which took place in his absence.