The witness (drasta), looks on passively but consciously, with acute alertness. It never participates in that which is looked at. It stands aloof and separate, a step away, as it were. The seen is 'there' and not 'here'. See the mind in operation. It is drsya, you are drasta. Look at it, don't deny or assert.
For example, I assume the attitude of a class teacher. I don't simply repeat to myself, "I am the class teacher", rather I act as such. I enter the classroom. Somebody talks loudly. I give him a stern look and he stops talking.
In the same manner, I don't tell myself that I am the onlooker. I act as such. Thoughts arise, float about, and move away. As I look on, they become shy and disappear. That is the test of whether I have truly assumed the attitude of an onlooker.