client/state/jetpack-sync/selectors.js
function isFullSyncing( state, siteId ) {
	const syncStatus = getSyncStatus( state, siteId );
	if ( ! syncStatus ) {
		return false;
	}

	const isStarted = get( syncStatus, 'started' );
	const isFinished = get( syncStatus, 'finished' );

	return isStarted && ! isFinished;
}
function isFullSyncing( state, siteId ) {
	const syncStatus = getSyncStatus( state, siteId );
	if ( ! syncStatus ) {
		return false;
	}

	const isStarted = get( syncStatus, 'started' );
	const isFinished = get( syncStatus, 'finished' );

	return isStarted && ! isFinished;
}
Returns a boolean for whether a site is in the process of a full sync.
@param  {Object} state    Global state tree
@param  {Number} siteId   Site ID
@return {Boolean}         Whether a sync is in the process of syncing