I'd advise practicing the maha mudra, as depicted in the Hatha Yoga Pradipika. It is the variation where you press lightly on the closed eye globes with the palm of the hands while doing fire breathing. This can be done in the lotus posture at the end of the practice session.

At the same time maintain the gaze through the location of the subtle eye (or as many would say the third eye). It is best done in my experience when practice is achieved in the brahma muhurta, and before exposure to artificial lighting.

The observed representation(s) will be a more succint and channelled indication of the capacity of the subtle eye than some other random observations.

It can allow a deeper and further exploration in this vein. However, the level of consciousness that can be accessed can also be unsettling on the initial attempts, as it will be most distinguished and unusual.