It is a matter of trying different things. Some responses are very basic reflexes. At the other extreme are behaviors such as complex conversations. Lights or objects that the person can see are another type. Familiar odors or different kinds of touch are also used.
Vestibular therapy involves moving the person around passively.
This provides touch as well as vestibular or positions sensation. Do the Coma Scales require an evaluation by someone with experience. A patient may be able to see, hear, smell etc. If there a motor impairments that may prevent the person from responding. In the extreme cases this is called Locked in Syndrome.