How would you test whether an antibody binds specifically to its intended antigen?

Why

Begin with authenticated antigen and appropriate positive and negative controls, including target knockout or low-expression cells matched for lineage where possible. Titrate the antibody rather than relying on one bright staining concentration. Verify binding on independent antigen preparations or a second detection format, and consider blocking the intended epitope with a well-characterized ligand.

Account for Fc receptor binding, secondary-antibody cross-reactivity, dead-cell staining, aggregates and polyreactivity to unrelated proteins or membranes. For example, if fluorescence remains equally high in a target knockout, apparent specificity is weak even when purified antigen ELISA is positive. A target knockout can also alter cell state, so include rescue or re-expression when practical.

A final specificity claim should be bounded by the assays, formats and antigen conformations tested.