Electro-mobility shift-assays (or gel retardation assays) are a standard way of assessing TF-binding. A fragment of DNA of interest is amplified and labeled with a fluorophore. The fragment is left to incubate in a solution containing abundant TF and non-specific DNA (e.g. randomly cleaved DNA from salmon sperm, of all things) and then a gel is run with the incubated sample and a control (sample that has not been in contact with the TF). If the TF has bound the sample, the complex will migrate more slowly than unbound DNA through the gel, and this retarded band can be used as evidence of binding. The unspecific DNA ensures that the binding is specific to the fragment of interest and that any non-specific DNA-binding proteins left-over in the TF purification will bind there, instead of on the fragment of interest. EMSAs are typically carried out in a bunch of fragments, shown as multiple double (control+experiment) lanes in a wide picture. Certain additional controls are run in at least one of the fragments to ascertain specificity. In the most basic of these, specific competitor (the fragment of interest or a known positive control, unlabelled) is added to the reaction. This should sequester the TF and hence make the retardation band disappear, proving that the binding is indeed specific

Target promoter is inserted in front of a GFP gene, and the target TF is induced. Expression levels of GFP should thereby depend on the target TF.

Visual inspection of promoter region to identify putative binding sites based on previous knowledge

X-ray crystallography is a method to determine the molecular structure of a crystal that is typically used to probe at the tridimensional structure of proteins. In some cases, it is possible to co-crystallize the protein bound to its DNA binding site, providing detail on the particular arrangement of the two components. Doing so provides, obviously, unequivocal evidence of binding.