In this paper we prove a sufficient condition, in terms of the behavior of a ground state of a symmetric critical operator P 1 , such that a nonzero subsolution of a symmetric nonnegative operator P 0 is a ground state. Particularly, if P j := −∆ + V j , for j = 0, 1, are two nonnegative Schrödinger operators defined on Ω ⊆ R d such that P 1 is critical in Ω with a ground state ϕ, the function ψ 0 is a subsolution of the equation P 0 u = 0 in Ω and satisfies |ψ| ≤ Cϕ in Ω, then P 0 is critical in Ω and ψ is its ground state. In particular, ψ is (up to a multiplicative constant) the unique positive supersolution of the equation P 0 u = 0 in Ω. Similar results hold for general symmetric operators, and also on Riemannian manifolds.