A procedure for the numerical computation of frequencies and amplitudes of quasi-periodic functions from equally spaced samples will be presented. It is based on a collocation-like strategy in frequency domain, using the Discrete Fourier Transform (DFT). Comments will be made on the practical choice of parameters in order to obtain high precision, avoiding DFT-related phenomena (leakage, aliasing). An application will be given to the study of the dynamics in the (practical) stability zone around the triangular libration points of the planar, circular RTBP for the Sun-Jupiter mass ratio. Joint work with G. Gómez and C. Simó