How early can doctors determine if a fetus has any birth defects from medications? and what test should be preformed ?

Depends on situation: Every newborn should have a thorough exam at birth looking for transient distortions caused by uterine compression, disruptions from intrauterine events, or abnormalities from whatever cause. If mom was on a specific medication, the sometimes hidden effects can be specifically looked for by those aware of the possibilities. There is no one test or group of tests.

Birth defects: It will depend on what organ of the body the defect will be located. A fetal ultrasound can do that for you.

Try not to stress!: Fus & fetal cardiac echo in the 2nd trimester show some anomalies caused by early exposures, as organs start forming as early as 3 weeks' conceptual age. Screenings of maternal serum in the 1st & 2nd trimester help define some risks. Some meds change the function of a gene, but not it's structure. Your OB & prenatal genetic counselor will advise you of risks & if more invasive tests are warranted.