Please refer the following steps:
1. Login to your server as root user.
2. Fire the following command:
root@mybox [~]# /usr/local/apache/bin/httpd -v
Server version: Apache/2.2.16 (Unix)
Server built: Oct 3 2010 11:41:41
OR
root@mybox [~]# httpd -v
Server version: Apache/2.2.16 (Unix)
Server built: Oct 3 2010 11:41:41
That should allow you to check the installed version of apache on your server.
