The ObjC API provides the cocos2d scene graph node and some convenience methods for basic usage. For advanced usage you will need to use the spine-c API. It isn't worth trying to wrap the whole C API in ObjC.
You can tell an animation has ended when the time > animation duration. So:
CCSkeletonNode* node = ...
if (node.state->time > node.state->animation->duration) {
// ended
}