First of all, can't launch Hot re-indexing via JMX if index is already in offline mode. It means that index is currently is invoked in some operations, like re-indexing at startup, copying in cluster to another node or whatever. Another important this is Hot Asynchronous Reindexing via JMX and "on startup" reindexing are completely different features. So you can't get the state of startup reindexing using command getHotReindexingState in JMX interface, but there are some common JMX operations:
getIOMode - returns current index IO mode (READ_ONLY / READ_WRITE), belongs to clustered configuration states;
getState - returns current state: ONLINE / OFFLINE.