How to know elastic search installed version from kibana?

from the Chrome Rest client make a GET request or curl -XGET 'http://localhost:9200' in console

rest client: http://localhost:9200

{
    "name": "node",
    "cluster_name": "elasticsearch-cluster",
    "version": {
        "number": "2.3.4",
        "build_hash": "dcxbgvzdfbbhfxbhx",
        "build_timestamp": "2016-06-30T11:24:31Z",
        "build_snapshot": false,
        "lucene_version": "5.5.0"
    },
    "tagline": "You Know, for Search"
}

where number field denotes the elasticsearch version. Here elasticsearch version is 2.3.4

I would like to add which isn't mentioned in above answers.

From your kibana's dev console, hit following command:

GET /

This is similar to accessing localhost:9200 from browser.

Hope this will help someone.