How can I tell what version of apache I'm running?

Either rpm -q httpd or /usr/sbin/httpd -v should work.

For recent Apache versions, try this:

$ /usr/sbin/apache2 -v

The output should be something like this:

Server version: Apache/2.4.7 (Ubuntu)
Server built:   Jul 24 2015 17:25:11

The above check only includes the primary version number, not including the extended backport patches.

If you installed with yum you can:

yum list httpd

and get the full version (note the -31 / -47)

Installed Packages httpd.x86_64 2.2.15-31.el6.centos Available Packages httpd.x86_64 2.2.15-47.el6.centos