I understand that new submissions to the showcase first must be verified ...
I sent in 2 URLs but only 1 was accepted. Both are SS sites.

How do you check if a site is made with SS?

There is a meta tag in the head part of your website's source code that identifies SS.

<meta name="generator" content="SilverStripe - http://silverstripe.org" />

If you have a website profiler installed, you can pull a lot of useful information about what a website's running on. See the attached screenshot of a profiler I ran on my client's site in Chrome. It can be pretty interesting going around looking into what others are using. =D

so they only accept sites with the "generator" meta tag :-)
I think they should check som /Security/* links

It's good practice to hide the used CMS.

I don't think /Security/ links are a reliable indicator of a SS-based website because any other framework could be using the same URL segments.

You can remove the generator meta tag if you like, I believe it's overridden in _config.php. Do a search for it on this forum - there's a thread of mine about it that got an answer from Willr.

I don't think revealing which CMS you're using is a problem. Many hugely popular sites are known to run on Wordpress and it's not a security issue. Sure, any hacker could go check out the source code and look for vulnerabilities, but it all really comes down to how well you've locked down your server and implemented security measures in your application.

I'm with you in case of security implementations.
When checking /Security/... links you can identify SS sites often how the forms are generated, I was meaning that.