From a DATYists point of view.
Breathing, color, body movements, vocalization etc are all indicators but there are a couple of dandies that to me are sure-fire indicators of orgasm. These both make use of the observation of involuntary muscle movements. Most of us are aware of the trembling of the limbs just before orgasm, which is a good example of this type of movement.
As a DATY nut, I am always cognizant of the abdominal muscles, which I can obviously keep an eye on. As the tension mounts, it is quite obvious when orgasm is near, for the muscles running vertically from the navel to the pubic area begin to knot up and become quite raised.
Another muscular event that is easily evident to a DATY nut, is that, as the vaginal muscles begin to spasm, so does the anal sphincter, which is intimately tied into the vaginal muscles and nerves. A fingertip kept on the anus during DATY is a good way to measure the pulse, so to speak, of the encounter. As the orgasm approaches, the spasms are readily felt there. When it actually happens the anus generally becomes very "mobile"(can't think of a better word at the moment).
P.S. maybe active is the word I was looking for.