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When I was in Ecuador (which uses the US dollar) it was extremely common to find fake $20s in your change. I could never spot them, but the locals could.

They were exceptionally good, right down to the watermark, security strip, etc. etc. Even when I had a known real one and a known fake one, I couldn't tell them apart.

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Well which part about the note made it a "known fake" to the locals?

They could feel the texture with their fingers (I couldn't tell any difference) and the security strip was a tiny, tiny bit longer than it should have been on the fakes.

Even after they told me what to look for, I couldn't tell.

This is precisely the reason the U.S. Treasury still insists on using natural fiber in paper notes. There's more detail in this delightful Esquire article about the making of paper currency:

http://www.esquire.com/features/benjamin-hundred-dollar-bill....

I can't seem to make your link work, but I managed to track down the original article so allow me to repost:

http://www.esquire.com/features/benjamin-hundred-dollar-bill...

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