You may use the receptor ligand binding assay in which a labeled compound, a ligand, which binds to a receptor, is used to detect that receptor. Usually, the ligand (a known protein) is labeled by means of a radioactive isotope, such as 3H, 125I, 35S, etc., but using a fluorescent moiety is also possible.
You need to bear in mind that non-specific binding is possible with high-affinity ligands. In addition, structural changes to the ligands can be introduced through the labeling process, which in turn may increase off-target binding.