Version information can be found on the About page of the Intranet. It’s usually a link in the lower right hand corner, right after the login. You can also find it by going to the More menu at the top of the page -> About Koha.
Viewing source (ctrl-u) on the OPAC also will give the version – it will look something like
meta name="generator" content="Koha 3.1700054"
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