Is there awareness of impermanence during the formless meditations Is there awareness of impermanence during the formless meditations?
If you choose to see things as they are or investigate the Dhamma. Since in the 8th Jhana there is a residue of Perception and Feeling hence you should direct your observation to understand the impermanence of feeling or impermanence of what is felt if you what to develop insight into the 3 marks of existance.

A person taking the Arupa Jhana route attains the final goal by going though Nirodha Samapathi (cessation of perception and feeling)

See: Raho,gata Sutta for further details.