Passive or Active Immunization?
Unfortunately, there is no definitive test to determine if anti-D is passive or active.
Tests can be done that will suggest whether the anti-D is probably passive or active. However, many laboratories do not perform such testing routinely since it's both safe and efficient to consider a post-RhIg anti-D to be passive without further testing, thereby triggering a post-natal RhIg injection. Further testing would be done if serologic test results suggest an immune anti-D, e.g., 4+ reactions with D+ red cells.
One test that may be used to try to interpret if anti-D is passive or active is antibody titration.