The best way to check for sickle cell trait is through a test that identifies which type of hemoglobin is present through HPLC. Sickle cell is an inherited condition in which both hemoglobin A and S are produced in the red blood cells, always producing more A than S. Individuals with sickle cell trait are generally healthy.
Understanding Your Sickle Cell Trait Screening Results
https://www.redcrossblood.org/donate-blood/dlp/sickle-cell-testing-details.html