  Fully characterizing the steerability of a quantum state of a bipartite
system has remained an open problem since the concept of steerability was
defined. In this work, using our recent geometrical approach to steerability,
we suggest a necessary and sufficient condition for a two-qubit state to be
steerable with respect to projective measurements. To this end, we define the
critical radius of local models and show that a state of two qubits is
steerable with respect to projective measurements from Alice's side if and only
if her critical radius of local models is less than $1$. As an example, we
calculate the critical radius of local models for the so-called T-states by
proving the optimality of a recently-suggested ansatz for Alice's local hidden
state model.
