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

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

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