How should a father approach a potential custody battle against the mother of his child.?

Recently my 7 month old son and I had our world turned upside down when his mother/my fiance a RN; was caught and later fired for stealing and abusing Rx drugs (dilaudid). Speculation about her abuse has surfaced several times during our three year relationship, however, was very cleverly disguised through lies and deceit. Despite any animosity I might have towards my fiance for what she has done, my primary concern is the safety and well being of my little boy; therefore, considerations must be made in regards to a custody battle. If anyone has any advice or wants to state their opinion it would be greatly appreciated. Thank you.