With a HTTP GET request to:
yoursolrhost:8983/solr/admin/info/system
you'll retrieve a structure in many different representations (xml, json, php, python,...).
the `lucene` key of the structure contains version numbers.
For example, using json output:
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The `lucene` key looks like this:
"lucene": {
"solr-spec-version": "4.6.0",
"solr-impl-version": "4.6.0 1543363 - simon - 2013-11-19 11:16:33",
"lucene-spec-version": "4.6.0",
"lucene-impl-version": "4.6.0 1543363 - simon - 2013-11-19 11:05:50"
},