You can check this by performing a curl -I on one of the files from your site. For example:
curl -I http://onapp.com/assets/img/sections/home/3.1-logo.png
HTTP/1.1 200 OK
Date: Tue, 19 Nov 2013 17:01:19 GMT
Server: Apache/2.0.64 (Unix) mod_ssl/2.0.64 OpenSSL/0.9.8e-fips-rhel5 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 PHP/5.2.9
Last-Modified: Tue, 12 Nov 2013 12:18:34 GMT
Accept-Ranges: bytes
Content-Length: 5621
Cache-Control: max-age=2592000, public
Expires: Thu, 19 Dec 2013 17:01:19 GMT
Content-Type: image/png
Under the "Expires" field, you can see when the content cache will expire. In this example, Thu, 19 Dec 2013 17:01:19 GMT.