You can include the If-Modified-Since header with a date in EEE, dd MMM yyyy HH:mm:ss z format when you use the SObject Describe resource. If you do, response metadata will only be returned if the object metadata has changed since the provided date. If the metadata has not been modified since the provided date, a 304 Not Modified status code is returned, with no response body.
The following example assumes that no changes, such as new custom fields, have been made to the Merchandise__c object after July 3rd, 2013.
HTTP/1.1 304 Not Modified Date: Fri, 12 Jul 2013 05:03:24 GMT
If there were changes to Merchandise__c made after July 3rd, 2013, the response body would contain the metadata for Merchandise__c. See Get Field and Other Metadata for an Object for an example.