It would look something like this
void DanceOnce ()
{
Spine.TrackEntry trackEntry = GetComponent<SkeletonAnimation>().state.SetAnimation(0, "dance once", false);
trackEntry.Complete += DoSomethingAfter;
}
void DoSomethingAfter (AnimationState state, int trackIndex, int loopCount)
{
Debug.Log("something after");
}
If you're new to coding in C#, you may have a bit of reading a head of you. Make sure you understand C# events and delegates.
Also, I recommend you read the Spine-Unity documentation if you want to understand something in particular:
General: http://esotericsoftware.com/spine-unity
Events: http://esotericsoftware.com/spine-unity-events