  Berwald geometries are Finsler geometries close to (pseudo)-Riemannian
geometries. We establish a simple first order partial differential equation as
necessary and sufficient condition, which a given Finsler Lagrangian has to
satisfy to be of Berwald type. Applied to $(\alpha,\beta)$-Finsler spaces,
respectively $(A,B)$-Finsler spacetimes, this reduces to a necessary and
sufficient condition for the Levi-Civita covariant derivative of the defining
$1$-form. We illustrate our results with novel examples of
$(\alpha,\beta)$-Berwald geometries which represent Finslerian versions of
Kundt (constant scalar invariant) spacetimes. The results generalize earlier
findings by Tavakol and van den Bergh, as well as the Berwald conditions for
Randers and m-Kropina resp. very special/general relativity geometries.
