How to check if search_index's solr is running?
The SolrConnector plugin provides a pingServer() function that will determine it the endpoint can be accessed, so to really test it you likely need to be able to call that.

I'd first check to see if there is an abled server by calling $solrIndex-&gt;isServerEnabled(). 

Then I'd get the server, its backend, the connector, and run the ping:

$server = $solrIndex-&gt;getServerInstance();
$backend = $server-&gt;getBackend();
$connector = $backend-&gt;getSolrConnector();
$response = $connector-&gt;pingServer();


or just 

if( $solrIndex-&gt;getServerInstance()-&gt;getBackend()-&gt;getSolrConnector()-&gt;pingServer() ){ 
  // Yup
} 
else {
 // nope
}


The return value from pingServer() is:


  The latency in milliseconds if the core can be accessed, otherwise
  FALSE.