I’ve only seen this episode once, so I couldn’t remember who was who. However, I’m proud to say that it took me only a few seconds to make the discernment. When the Kirk on the left talks to Spock, he keeps his gaze on the Vulcan. When the Kirk on the right talks to Spock, he keeps his gaze on the other Kirk. That’s when it clicked. Kirk and Garth are experienced at combat; they know to keep their eyes on what is threatening them. For Kirk, the threat is Garth. For Garth, the threat is Spock. Therefore, the Kirk on the left is Garth and the Kirk on the right is the real Kirk. Boom. Beat that, Spock.