Two options that spring to my mind:
yum info httpd
check the output to see if it is installed or not, you get something like:
[0 1003 12:18:33] ~ % yum info httpd
Loaded plugins: fastestmirror, refresh-packagekit, security
Loading mirror speeds from cached hostfile
* base: ftp.halifax.rwth-aachen.de
* extras: ftp.rrzn.uni-hannover.de
* remi: mirror5.layerjet.com
* remi-php56: mirror5.layerjet.com
* remi-safe: mirror5.layerjet.com
* updates: ftp.halifax.rwth-aachen.de
Installed Packages
Name : httpd
Arch : x86_64
Version : 2.2.15
Release : 47.el6.centos
Size : 2.9 M
Repo : installed
From repo : updates
Summary : Apache HTTP Server
URL : http://httpd.apache.org/
License : ASL 2.0
Description : The Apache HTTP Server is a powerful, efficient, and extensible
: web server.
or do
rpm -qf /etc/httpd
to see if any package thinks it's responsible for httpd's config directory – on CentOS6, you get
[0 1001 12:12:46] ~ % rpm -qf /etc/httpd
httpd-2.2.15-47.el6.centos.x86_64
(and easiest last, convention on RH/CentOS is that the config lives in /etc/httpd/, so its mere presence might be a bit of an indicator.)