Sandfish skinks exhibit only mild sexual dimorphism, so telling the difference between the genders can be a little tricky. But compared to females, males tend to have brighter, higher contrast coloring. They are also generally larger than females, and thicker at the base of the tail.
Potential male S. scincus
"Scincus scincus" by Alex Slavenko, licensed under CC BY-NC-SA 2.0
Potential female S. scincus
Photo by basem, licensed under CC BY-NC 4.0