I would like to know, what is the fastest way to check if a question is changed using the API calls.  
For my purpose (a cache), a question is considered changed if:

- A new comment is added to the question or to any of the answers
- It had any new [activity][1]

What is the fastest way to get this check?

<sup>* the check should work also for questions with hundreds of answers</sup>



  [1]: http://stackapps.com/questions/1742/last-activity-date-meaning-on-questions-route