How do I know if the custom theme is a child theme or a full-custom theme?

Question

Inherited a custom built WordPress site. How do I know if the theme it uses is a child theme or a full-custom theme? I would like to delete the unused themes for security and to free up space, but have a nagging worry that one of them may be a parent theme. It seems like one of those situations where if it is a child theme, I would see evidence of such -- I don't see any mention of a parent theme anywhere in my CSS or theme files. The theme folder has four themes available -- vsl-new (my theme; active; no mention of a parent), Twenty Twenty-one, Twenty Twenty-two, and Twenty Twenty-three. I don't want to assume anything tho. Is there a way to know for sure either way?

Answer 1

If it's a child theme, the theme's style.css file will refer to its parent:

/*
Theme Name:   Twenty Fifteen Child
// ...
Template:     twentyfifteen
*/

That Template: line will tell you which theme is the parent. If there's no Template: line, then this theme is not a child theme.

Reference: Child Themes in the WP Developer site