  For the phenomenological description of magnetohydrodynamic turbulence
competing models exist, e.g. Boldyrev [Phys.Rev.Lett. \textbf{96}, 115002,
2006] and Gogoberidze [Phys.Plas. \textbf{14}, 022304, 2007], which predict the
same Eulerian inertial-range scaling of the turbulent energy spectrum although
they employ fundamentally different basic interaction mechanisms. {A relation
is found that links} the Lagrangian frequency spectrum {with} the
autocorrelation timescale of the turbulent fluctuations, $\tau_\mathrm{ac}$,
and the associated cascade timescale, $\tau_{\mathrm{cas}}$. Thus, the
Lagrangian energy spectrum can serve to identify weak
($\tau_\mathrm{ac}\ll\tau_{\mathrm{cas}}$) and strong
($\tau_\mathrm{ac}\sim\tau_{\mathrm{cas}}$) interaction mechanisms providing
insight into the turbulent energy cascade. The new approach is illustrated by
results from direct numerical simulations of two- and three-dimensional
incompressible MHD turbulence.
