Both theoretical and experimental analysis of a new efficient method to measure the number and type of modes propagating in optical fibers are presented. This consists in measuring the intensity of the near field image at the end of a fiber while scanning the wavelength with a laser source signal. Modes are extracted from Fourier transformation of the spectral data at each point (x,y) of the images. A novel technique which is referred to as scalar product technique is implemented in order to reconfirm real modes and exclude spurious modes. The technique is based on the orthogonality of different modes. A standard multimode fiber has been measured to verify the technique. Three real modes LP01, LP11, and LP02 are discovered and reconfirmed by the orthogonality with the minimum values of the scalar products. One spurious mode, which comes from the dependence of the power of the laser source on the wavelength, is thus excluded as it is not orthogonal either to the LP11 mode or the LP02 mode.