  Brillouin light scattering spectroscopy from so-called standing spin waves in
thin magnetic films is often used to determine the magnetic exchange constant.
The data analysis of the experimentally determined spin-wave modes requires an
unambiguous assignment to the correct spin wave mode orders. Often additional
investigations are needed to guarantee correct assignment. This is particularly
important in the case of Heusler compounds where values of the exchange
constant vary substantially between different compounds. As a showcase, we
report on the determination of the exchange constant (exchange stiffness
constant) in Co$_2$MnSi, which is found to be $A=2.35\pm0.1$ $\mu$erg/cm
($D=575\pm20$ meV \AA$^2$), a value comparable to the value of the exchange
constant of Co.
