How can the government tell if money is counterfit?

Question

How can the government tell if money is counterfit?

Answer 1

in the US:

the paper is made of cotton, linen and satin in an undisclosed ratio. There are blue and red nylon fibers dispersed throughout the media.  Getting "the feel" of money correct is hard for counterfeiters to do.

The paper is also watermarked.  

There is rarely any solid shading or lines. This make it hard for normal printers to reproduce because of the extremely high resolution.

It is printed with an intaglio printing press, which raises images and text rather than pushing them into the media.

The holograms are hard to duplicate and printed with the special ink.

There are now also UV metal strips implanted in some denominations that will light up under a black light.

If any of these are missing, the government can tell.

Answer 2

When you feel the cuffs go on you know it wasn't realistic enough.

Answer 3

I work in retail and we have an electronic device that we put bills in to check for authenticity.  The special light detects certain watermarks in the bills and each denomination is different.  Those marker pens are obsolete now.

Answer 4

There are a lot of little markers hidden in the engraving. It is made out of special paper with colored threads woven in. And there is a metallic strip in most bills that is specific to that denomination.