Ensuring that Solr is meeting the SLA expectations for performance is the goal of monitoring. Solr provides both RESTful and JMX hooks to allow you to integrate Solr into your enterprise monitoring platform.
Note
Don't have your own monitoring platform? There are two offerings, available from New Relic (http://newrelic.com) and Sematext (http://sematext.com/spm/) that provide a comprehensive monitoring solution. Both are cloud based (SaaS) and communicate via a small agent installed into Solr that provides a wealth of statistics and analysis about the JVM, as well as Solr specific metrics such as request response time and throughput, cache hit rate, and indexing performance.