Thank you for your response, Nobbie. Let me start by answering your questions:
Did you install Xampp or did you install Apache somehow from OSX?
Don't you remember what you installed?
I installed Xampp 5.6.20 about a year ago and have been using it successfully since that time. I did not "install" Apache from OS X. OS X ships with the Apache files pre-installed - that is the source of the confusion here. Installing Xampp apparently installs a second set of Apache files.
To use the Apache shipped with OS X, one must configure it and then start it with the apachectl command. I have not done that. To start the Apache in Xampp, I use the Application Manager that ships with Xampp. This works great.
Enter "which httpd" in a terminal, what is the output?
There is no Apache splash screen, what do you mean by that? Apache is a server, not a GUI.
I've been Web programming for about 20 years - I understand what a server is.
Let me restate the question: I am successfully using XAMPP 5.6.20. I want to start using the Apache VirtualHost feature, which requires changing the Apache configuration file. If I look for that file, I find two different files: one is /etc/apache2/httpd.conf and the other is /Applications/XAMPP/xamppfiles/etc/httpd.conf. Which one should I modify?
Since one configuration file is in an apache2 folder, I thought that asking Apache what version is actually running would point to the configuration file to modify. I naively thought that the Apache version in Xampp might be 1.3.x or something. I see now that that is obviously not the case: both the Apache shipped with OS X and the Apache running in Xampp are 2.x.x.
So the answer to my original question is now obvious: since I'm successfully running Xampp and have not "started" the Apache shipped with OS X, I should configure Xammp's own Apache configuration file in /xamppfiles/etc/httpd.conf.
Further color on finding an application's version number: One could just look for the version number in the application's file-system metadata, but this may not be accurate in the case where there are multiple copies of the application in the file system. So the most reliable way is to ask the running application itself what its version number is.
For example, if I want an accurate answer to the question "what version of PHP am I running?" I simply execute the PHPInfo() function. It answers "PHP Version was 5.6.14."
Note that the "httpd -v" command and the "which httpd" command apparently just get the version number from the file system - they do not get the version number by asking the running copy of Apache (those commands work whether Apache is running or not).
Xampp's Application Manager does not have a feature that indicates what version of Apache Xampp contains and is launching. So in the original question, I was asking "how do I ask the running copy of Apache what version it is?" A splash screen would be a common way to find out, but as you say, Apache doesn't have one.