Step #1: Find out if response has a Last-Modified header

Type the following curl command:

curl --silent --head http://server1.cyberciti.biz/foo/bar/image.png
curl --silent --head http://server1.cyberciti.biz/foo/help.html

OR

curl -I http://server1.cyberciti.biz/foo/bar/image.png
curl -I  http://server1.cyberciti.biz/foo/help.html

In this example, note down the Last-Modified headers in the response to this HEAD request:
$ curl -I http://www.cyberciti.biz/faq/
Sample outputs:

HTTP/1.1 200 OK
Server: nginx
Date: Tue, 11 Dec 2012 10:10:24 GMT
Content-Type: text/html; charset=UTF-8
Connection: keep-alive
X-Whom: l2-com-cyber
Last-Modified: Tue, 11 Dec 2012 10:10:23 GMT
Cache-Control: max-age=299, must-revalidate
Vary: Cookie
X-Pingback: http://www.cyberciti.biz/faq/xmlrpc.php
X-Galaxy: Andromeda-1
Vary: Accept-Encoding

The syntax is as follows to send If-Modified-Since header using the curl command line:
$ curl -I --header 'If-Modified-Since: DATE-FORMAT-HERE' http://server1.cyberciti.biz/foo/bar/image.png
$ curl -I --header 'If-Modified-Since: Tue, 11 Dec 2012 10:10:24 GMT' http://www.cyberciti.biz/faq/
Sample outputs:

HTTP/1.1 304 Not Modified
Server: nginx
Date: Tue, 11 Dec 2012 10:12:11 GMT
Connection: keep-alive
X-Whom: l2-com-cyber
Vary: Cookie
Last-Modified: Tue, 11 Dec 2012 10:10:23 GMT
X-Galaxy: Andromeda-1
Vary: Accept-Encoding

The resource sends a 304 Not Modified response, indicating that it supports Last-Modified validation.