A scheme of analysis for the dynamic structure functions in pure liquids is presented which can be implemented with both experimental and simulation data. Expressions for contributions of relaxing and propagating modes proposed earlier in the framework of the generalized collective modes approach are optimized in order to strictly fulfil three among the required sum-rules. The method is applied to simulation data for liquid cesium, the description of which appears to only require one relaxing and one propagating mode in the investigated wavevector range. These expressions are able to account for the dynamics in both the hydrodynamic and the kinetic regimes, being quantitatively accurate up to the onset of the first peak of the static structure factor and qualitatively beyond. Features of the modes can thus be obtained easily, without resorting to heavy formalism. The scheme of analysis can be straightforwardly extended to account for a higher number of relaxing and propagating modes.