Lyapunov function - double pendulum

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I deal with analyzing the double pendulum system. in the first stage, I found the motion equation. now, I would like to find the stability condition. I want to do that by using Lyapunov stability theorem I know that there are 3 conditions that should exist for the Lyapunov function: $V(x')=0$, $V(x)>0$, $V'(x)\leq0$. My question is, how can I know which Lyapunov function to choose? Guessing? trial and error? thanks in advance