To monitor Solr start by enabling JMX (see SolrJmx).
Tools & Services
SPM for Solr
SPM for Solr from Sematext (http://sematext.com/spm/solr-performance-monitoring/index.html) monitors all Solr metrics, as well as a number of server and JVM metrics. It includes alerting, multiple custom dashboards, custom metrics, subscription emails, filtering by server, core, request handler and more. SPM can monitor Solr 3.x and 4.x servers/clusters, including SolrCloud. In addition to monitoring Solr, SPM can monitor Hadoop, HBase, ElasticSearch, Sensei, Java webapps and generic Java applications, as well as custom metrics. SPM can be installed on premises or one can use the SaaS version of SPM run by Sematext in which case the setup takes less than 5 minutes before graphs with performance metrics start appearing in real-time.