Before we get into the actual topic, we need to understand what is an agent-less check and later delve into the answer to the question, what should we look for over the network? We define agent-less check as the Solr health check, which can be performed remotely.
The first and the foremost thing we should do is to have a host-level check, which can be expected to perform a network-level ping. Then, we may go for checking TCP connectivity to the servlet container port and then make an HTTP GET request to the Solr front page checking for a known string (for instance, Welcome to Solr Monitoring and Optimization).
Now we can start with performing Solr specific checks by monitoring items which may include: