Inferior Bounds for Phase Synchronization
  An approach is presented for coupled chaotic systems, estimating an inferior bound value for the absolute phase difference, in order to say that phase synchronization is present. This approach shows that synchronicity in phase implies synchronicity in the time of events. This is used to derive an equation to detect phase synchronization, based on the absolute difference between the time of these events. A phase based on the vector field is also introduced. It can be used to calculate the frequency of attractors with non-coherent phase dynamics. 
M.S. Baptista, T. Pereira and J. Kurths