What I do is build this concept directly into the web service API. If a client sends or requests data in a format (media type) that is no longer supported by the server, the server responds to that request with a status code (e.g. 415 for a RESTful web service). Likewise, if the client tries to invoke a method that is no longer supported or access a resource that no longer exists (but should, as far as the client is concerned), then appropriate status codes are returned in those cases. My Android client interprets any of these status codes as indicating that it must be out of date with respect to the web service, and notifies the user that they should upgrade.
Therefore, there is no need for a special "am I still supported?" request.