A local injection of anesthetic agent into the sinus tarsi is a great diagnostic tool. In sinus tarsi syndrome, this injection will relieve pain (albeit temporarily) Sometimes it's a process of elimination so if the injection didn't change your symptoms then in theory you can rule out sinus tarsi syndrome.

Also in answer to one of your queries: Sinus tarsi syndrome does often leave you with a stiff subtalar joint, however a stiff subtalar joint can be a symptom of other conditions as well.
Additional Comment I forgot:
Oh and by the way - the article that uses STS and lateral impingement interchangeably isn't correct. They are two different syndromes!! Oh the internet.........