Which hook for a warning message?
Traditionally that's what hook_requirements() is for


  Check installation requirements and do status reporting.


Emphasis mine. 

It's how modules declare that they are missing a required Javascript library, for example.

It hooks your module into the status report at /admin/reports/status.

Drupal doesn't have a mechanism for displaying messages in response to a requirement failure; so if you need to show something to the user, hook_node_view() makes as much sense as anything else IMO.

Actually that's not strictly true (I didn't read the docs properly)...hook_requirements() does already handle that in one way or another. From the docs relating to the 'runtime' phase:


  any requirement with a severity of REQUIREMENT_ERROR severity will result in a notice on the administration configuration page.


So if your users have access to the admin config page you can use this hook to automatically display them a message. But if the message needs to be on a different page, you'll have to take a different route with the advice in @FreeRadical's answer.