First it depends what you mean by "XWiki version".

Version of some feature/API of XWiki

If what you want to know is which API/feature to use depending on the version then the safest is to check the version of the core or installed extension which contain this API/feature.

For example to know the version of Extension Manager scripting API you can do the following:

• Go to the Administration -> Extensions section -> switch to Installed or Core extensions and search for Extension - Script
• Create a new page or edit any existing page in wiki model, put the following and click the 'preview' button:
  {{velocity}}
  $services.extension.core.getCoreExtension('org.xwiki.platform:xwiki-platform-extension-script').id.version.value
  {{/velocity}}
• From Java:
  @Inject
  private CoreExtensionRepository coreExtensionRepository;
  ...
  CoreExtension extension = this.coreExtensionRepository.getCoreExtension("org.xwiki.platform:xwiki-platform-extension-script")
  Version version = extension.getId().getVersion()
• Access your server's filesystem and inside XWiki's WEB-INF/lib directory look for JARs using the format xwiki-platform-extension-script-<version>.jar. The <version> part will be your version.

Version of current product/distribution

The XWiki Dev Team maintain a XWiki distribution which is the default one and is in sync with the XWiki jars it contains. But anyone can create a distribution of XWiki (usually a custom WAR or something containing a WAR) by picking up some specific core extension. For example some service hosting special version of XWiki.

• Check the bottom of any page and if you haven't overwritten the default skin/templates you should see something like:

  

• Create a new page or edit any existing page in wiki model, put the following and click the 'preview' button, it will display your XWiki version:
  {{velocity}}
  $services.extension.core.repository.environmentExtension.id.version.value
  {{/velocity}}
• From Java:
  @Inject
  private CoreExtensionRepository coreExtensionRepository;
  ...
  CoreExtension distributionExtension = this.coreExtensionRepository.getEnvironmentExtension()
  Version version = distributionExtension.getId().getVersion()
• Access your server's filesystem and open XWiki's META-INF/extension.xed. Looks at the value of the <version> XML element (usually on the 4th line).