How do we know that a student is ready to attempt a more progressive posture such as Sarvāṅgāsana?
From following the core principle in the teachings of Vinyāsa Krama. In that the Pratikriyā Āsana or opposite action posture for a particular Āsana needs to be mastered before that particular Āsana is attempted.
So the student first works around Bhujaṅgāsana within their personal practice and the information that arises guides the teacher as to their readiness for, in this case, Sarvāṅgāsana.
“Teach what is inside you, not as it applies to you,
but as it applies to the student.”
– T Krishnamacharya
The information arising from observing how the student practices Bhujaṅgāsana guides the teacher as to the appropriateness of Sarvāṅgāsana. The information that feeds back may be on the level of Annamaya, Prāṇamaya, Manomaya or beyond. Obviously this implies that we are observing the students practice directly.
Once the student shows an adequate performance of Bhujaṅgāsana and it can be integrated into their existing personal practice, then we can be more secure that the student is ready to approach integrating Sarvāṅgāsana into their regular practice.