Short version - you can see that information in the response HTTP headers:

> **X-RateLimit-Current:298**  
> **X-RateLimit-Max:300**  

More info in the answer of [Is there a way of querying how much of the API limit has been used?][1].


  [1]: http://stackapps.com/questions/71/is-there-a-way-of-querying-how-much-of-the-api-limit-has-been-used