for example if you put a fake dollar in a vending machine, does it accept?

When I put a dollar into a pop/snack machine what does it check for on the dollar to make sure its a “legit” dollar bill? Links would be great! Thanks, Muah.

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I’m working on a school project where I make a circuit board to emulate a snack machine and I’m wondering how to make a sensor that does this… but I don’t know how to tell a one from a five without using some kind of image scanner/filtering program and thats obviously not how it works [...]