As an admin: 
Type in an internal URL (for example into the "Upload .ics file and show events" field in the oPluginsPanel admin page.
Click "Show events".
As a contributor: 
Create a post with the following shortcode: [booking-manager-listing url='' from='any' until='any']
And then preview the post.
As a subscriber: 
Use the parse-media-shortcode ajax action built in to WordPress to render the same shortcode as above. Here's an example using curl:

% curl -s -c .cookies -F log=subscriber -F pwd=password ''
% curl -s -b .cookies -d 'action=parse-media-shortcode' --data-urlencode "shortcode=[booking-manager-listing url='' from='any' until='any']" ''|jq .data.body
"<strong>[WPBM Error]</strong> File does not contain events <script src='' id='jquery-core-js'></script>\n<script src='' ...

Using timing or the responses from the various methods above, it's possible to determine if a port is open, and if it responds to http requests.