The only forensic analytical techniques that were used were when the autopsy was performed. There was no need for any DNA test or fingerprinting in these cases. As mentioned several times before, Nannie wasn't revealed until the eleventh murder of Sam Doss. His death was suspicious so an autopsy was performed on him and they found tons of a mysterious substance in his system. The toxicologist determined that it was arsenic that was in the body. Nannie was the only one to see Sam so they knew it was her. Then they discovered that she was responsible for all the murders of the family. The bodies of those victims were brought back and the poison was in them also.