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Say I do business with somebody and they pay me with a $100 bill. I want to know if it's real or not. I've already held it up to light to inspect the fibers of the paper, but I want to know if there's a hack to ensure it isn't counterfeit.

A method that works similarly to a counterfeit detection pen would be the best.

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I recently looked up a similar question myself and I can across a way to make your own counterfeit money pen (if it goes one colour it's real, the other other colour it is fake type thing).

What you need:

  • An invisible ink pen (you can get a highlighter and rinse it out so there is not colour left in the reservoir or in the nib)
  • A syringe (can be obtained online if you don't have any lying around)
  • Iodine (which is an antiseptic so it can be easily obtained from any drug store / pharmacy)

What you'll need to do:

  • Make sure the felt tip marker is free from any ink
  • Take 2-3ml of the iodine into the syringe (this may take a little while)
  • Inject the iodine directly into the tip of the marker or highlighter until it starts to ooze out
  • Wait for it to get absorbed by the marker and then repeat a couple of times

And there you have it, you can get a bank note and try marking it - if it comes out yellow/amber/brown, it's genuine, if it marks the note black, it's a dud. See image below (the small crosses are the pen marks)

enter image description here

Please note that iodine does stain so only make a small mark on your note as to not completely ruin it!

Here are more detailed instructions from where I found the idea before!

  • Love this. Will accept after trying. – Mooseman Mar 10 '15 at 11:26
  • It should be noted that the Secret Service, among others, say that this method is not very effective. It could give a false sense of security for "good" counterfeits, which are the ones that need the most checking. – TIO Begs Mar 11 '15 at 15:03
  • 1
    This tests the material of the paper, which can be circumvented by bleaching a dollar bill and reprinting a $100 bill over it (don't do this). – user3932000 Mar 14 '15 at 16:19
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It would perhaps be wise to purchase a scanner if you are in a high risk industry like a bar or a club. This will at least let your cashiers quickly check notes reasonably well in the variable lighting conditions.

Otherwise I do not believe there is a hack to your question. A study of the Secret Services "Know your money" website is your best path to becoming familiar with the US currency nuances.

EDIT: Some background, I used to exchange US dollars in large numbers some time back. And quite often I would receive in a batch of $100 bills one or two fake notes. Fortunately my money changer were understanding and sorted it out on my behalf. This was in Singapore not the US FYI.

However they recommend UV and Magnetic scanners as the only reliable way of identifying currency that is either in need of checking or outright fake.

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The list of publicly known US Currency security features can be found here.

Essentially, they are:

- Each bill has a watermark featuring the same historical figure as the main portrait.
- Use of Color-Shifting Ink on the face in the lower-left corner
- Various Fine-Line printing patterns
- Polyester security thread running through paper

The brand-new (late 2013) bills have completely new designs and sets of security features that can be found here. They are numerous and vary by denomination, so I'll leave them as an exercise for the reader.

If you don't wish to memorize these attributes, you can get an actual 'counterfeit detection pen' for under $5 here. They likely don't work any better than the hacked-one in the currently accepted answer, but I doubt you could re-create one for less.

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