Blood Screening
From Star Trek : Freedom's Wiki
Blood screening is a medical test used by Federation personnel to identify shape-shifters. This was considered necessary because of the possibility that shape-shifting Founders had taken humanoid form and infiltrated the Federation and other Alpha Quadrant powers.
The test involves removing a small sample of blood from an individual's body. If the individual is a changeling, the blood would revert to a gelatinous orange fluid.
