  Let $\Lambda$ be a row-finite higher-rank graph with no sources. We identify
a maximal commutative subalgebra $\mathcal{M}$ inside the Kumjian-Pask algebra
${\rm KP}_R(\Lambda)$. We also prove a generalized Cuntz-Krieger uniqueness
theorem for Kumjian-Pask algebras which says that a representation of ${\rm
KP}_R(\Lambda)$ is injective if and only if it is injective on $\mathcal{M}$.
