English equivalent of "a**holes don't have horns"

We have this expression in Hindi slang. Situation:

A: "I can't believe he treated me like that. I was stupid to trust him"

B: "Don't beat yourself up. How were you supposed to know he would stoop so low? After all, a**holes don't walk around with horns on their heads"

What B is trying to say is that bad people don't have distinctive physical characteristics. They look like normal people and there's no way to tell them apart.

Is there some English expression that conveys this?

Solution 1:

I suggest this old saying by Tucker Max "How were you supposed to know he would stoop so low? After all, the devil.......

Tucker Max - an American author and public speaker.

Solution 2:

Area, the only phrase I can think of that's like this is, you sometimes hear variations on "sign on their head" or "sign hanging over their head" or "sign over their head..."

So, "assholes don't go around with signs over their head" or "I wish idiots had a sign over their head alerting me to the fact they were idiots.." sort of thing.

As I mention in a comment, it's absolutely understandable. And, I'm pretty sure I've actually heard someone say something pretty much like this (using the word "horns") in English. So, you know, something like "the devil doesn't go around with horns and a tail!"

{Footnote: My grandparents etc are Scottish and I get some subtle linguistic clues from them. it's just possible there is some connection there since, as you know, there are many connections to Scottish usage, accent, etc in the subcontinent. perhaps there's a scottish saying along the lines of devils not always coming with handy visible horns (that would be typically miserable of scots grandmothers!)}

Solution 3:

Shakespeare said that someone (Iago, I think) "could smile and smile and be a villain", also that "There's no art to find the mind's construction in the face." (Macbeth) Can you use that?

The Hindi expression is a good one, we should steal it.

Solution 4:

A phrase that came to mind was Evil doesn't advertise but apparently it isn't widely used, nor is The Devil doesn't advertise, so I don't know if I'm misremembering something there.

While we're on Shakespeare however, from Hamlet,

the devil hath power to assume a pleasing shape