  Let $\Lambda$ be a dominant integral weight of level $k$ for the affine Lie
algebra $\mathfrak g$ and let $\alpha$ be a non-negative integral combination
of simple roots. We address the question of whether the weight
$\eta=\Lambda-\alpha$ lies in the set $P(\Lambda)$ of weights in the
irreducible highest-weight module with highest weight $\Lambda$. We give a
non-recursive criterion in terms of the coefficients of $\alpha$ modulo an
integral lattice $kM$, where $M$ is the lattice parameterizing the abelian
normal subgroup $T$ of the Weyl group. The criterion requires the preliminary
computation of a set no larger than the fundamental region for $kM$, and we
show how this set can be efficiently calculated.
