How can I view invalid characters in a (text) file?

At the left is Notepad++ on a Windows machine. At the right is TextWrangler on a Mac machine. Notepad++ shows me what those invalid characters are, but TextWrangler just displays inverted question marks. How can I view those invalid characters (more meaningfully) on Mac?

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OS: macOS High Sierra 10.13.3 Hardware: MacBook Pro Mid-2015

  • What makes a character invalid? – bmike Mar 12 '18 at 2:49

Sublime Text can display the invalid characters.

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