The answer would be skeletal markers and DNA analysis.
Skeletal markers are often used to identify the biological gender of skeletal remains. The most commonly used skeletal markers can be observed in the skull and in the pelvic bone.However, the accuracy of his method is limited to adults only the differences between male and female skeletal markers usually become visibly only after maturation.
DNA analysis is more widely uses nowadays because with the technology of today, laboratories can now detect and X and Y chromosome-specific sequences, which help them identify the biological gender.