To make sure it is on (and disable it if you want), you will need to go to the admin panel of your instance and enable / disable it there. In particular, you need to go to:
Administration> General Configuration> Install Accept Remote API Calls to Enable / Disable
To check if the REST api is enabled, just go (change the url to yours).
https://jira.atlassian.com/rest/api/2/user
and see if the page is loaded.
I do not think there is an API to get the version of the API, but you can make API calls and return in case of an error. Coding depends on what you are trying to achieve.
Further information on the REST API can be found at:
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