  Shock induced star formation in a stellar density wave scenario produces an
azimuthal gradient of ages across the spiral arms which has opposite signs on
either side of the corotation resonance (CR). We present a method based on the
Fourier analysis of azimuthal profiles, to locate the CR and determine the arm
character (trailing or leading) in spiral galaxies. Basically, we compare the
behavior of the phase angle of the two-armed spiral in blue and infrared colors
which pick out respectively young and older disk stellar population. We
illustrate the method using theoretical leading and trailing, spirals. We have
also applied the method to the spiral galaxies NGC 7479, for which we confirm
the reported leading arms, and NGC 1832. In these galaxies we find two and
three CRs respectively.
