The Navier-Stokes equations, after application of the Reynolds' averaging procedures, are the basis of direct surface-based measurements of turbulent fluxes via the eddy correlation method. Under restrictive conditions in the atmospheric surface layer, these equations are valid in a simplified form. These conditions are the stationarity of the data, the homogeneity of the underlying surface, a fully developed turbulence, and negligible density fluctuations. For most experiments these conditions are not fully satisfied, especially for continuous measurements over a greater period of time. Possible test algorithms are described to reflect the fulfilment of the criteria given above. As a consequence, we present a scheme for the characterization of the quality of direct turbulence measurements.