  An approach to determine the kinetic beam energy at the $10^{-5}$ level is
presented, which corresponds to an improvement by more than one order of
magnitude compared to conventional methods. Particularly, collinear
fluorescence and resonance ionization spectroscopy measurements on rare isotope
beams, where the beam energy is a major contribution to the uncertainty, can
benefit from this method. The approach is based on collinear spectroscopy and
requires no special equipment besides a wavelength meter, which is commonly
available. Its advent is demonstrated in a proof-of-principle experiment on a
Ni beam. In preparation for the energy measurement, the rest-frame transition
frequencies of the $3d^9 4s\;^3\mathrm{D}_3 \rightarrow 3d^9
4p\;^3\mathrm{P}_2$ transitions in neutral nickel isotopes have been identified
to be $\nu_0(^{58}\mathrm{Ni})=850\,343\,678\,(20)$ MHz and
$\nu_0(^{60}\mathrm{Ni})=850\,344\,183\,(20)$ MHz.
