Red Eyes, Take Warning
An easy way to tell a villainous (or sinister) character apart from the protagonists, other than giving them a Red Right Hand, is making their eyes red. If they don't start out evil, there's a chance they become so over time or perform full-blown Face–Heel Turn. It's not the same thing as the "red eye" phenomenon in photos, but if cameras could detect evil, or danger these would be the first people they'd catch.
Even those who aren't truly evil or dangerous are often thought to be so by characters In-Universe.
In the progressive algorithm of "Uh-Oh" Eyes we have:
Supernatural Gold Eyes < Red Eyes, Take Warning < Hellish Pupils < Glowing...read more