For the purpose of inheriting from a deceased father's estate, being included in a deceased father's will, or petitioning for Social Security Survivors Benefits or citizenship status, you may need to prove paternity. However, courts largely do not accept postmortem paternity tests as evidence. The law puts much emphasis on finality and allowing postmortem paternity tests undermines finality. However, this is an evolving area of law, largely because of evolving technology in DNA testing.
Locate a personal object of the deceased such as hair brush. A private agency can run paternity tests using hairs of the deceased for about $300. The results will not be admissible in court under any circumstances because there is no chain of custody on the DNA sample.
Request a court order to harvest a tissue sample from the deceased person. The time frame to extract a tissue sample is very limited. If you are even considering attempting to establish paternity after death, act before the person is buried.
Request a court order to exhume the body. The courts grant this request only in the most exigent circumstances such as for the purpose of solving a homicide. It will be extremely difficult to persuade the court to exhume a body for a paternity test.
Conduct a DNA test on a sibling or parent of the deceased. A DNA test on a sibling will be able to show that you are a child of the person's relative. Imperfect match DNA samples are almost never admitted in court. You are unlikely to get a court order to do the test if a relative does not consent.