There is multiple ways to find this but more accurately would be;
Login to web server
Go to apache instance and bin folder
Executed httpd with -v to get the version details.
[root@lab sbin]# ./httpd -v
Server version: Apache/2.2.15 (Unix)
Server built: Jul 18 2016 15:24:00
[root@lab sbin]#
Alternatively, you can also use the rpm command to check the installed version:
[root@lab ~]# rpm -qa |grep httpd
httpd-2.2.15-54.el6.centos.x86_64
httpd-tools-2.2.15-54.el6.centos.x86_64
[root@lab ~]#
