Do you mean Lyapunov exponnent? In that case one should use the Kattz or Rosenstein methods to estimate the maximal Lyapunov exponnent from the time series.
Wang Huiha,i Sun Kehui, He Shaobo. Characteristic analysis and DSP realization of fractional-order simplified Lorenz system based on Adomain decomposition method. International Journal of Bifurcation and Chaos, 2015, 25(6): 1550085.