damping envelope

So I'm trying to measure the dampening envelope of a variable mass compound pendulum, the pendulum is interfaced with labview (via a potentiometer). I've the data in an array and have a formula node set up so i can run a mat-script. Now in theory I just need to detect a point so that n1<n2>n3. ie the peak points will have points to either side of them that are or lesser magnitude. Unfortunately im badly out of practice with this and i dont really know where to even start. Any hep/suggestions will be greatly appreciated. Thank you.