as far as american bills go, there are 3 main ways of telling when one is a fake.
1. color shifting ink in the lower right corner. if it doesn't shift, it's not real.
2. watermark on the right of the portrait, visible when held up to light. the portrait in the watermark should match the one on the bill.
3. there's a security strip that repeats the nominal value of the bill. the placement is different for each bill (except $1s), and when put in UV light, they glow.
There are other ways to tell when a bill is fake including the texture of the paper and microtext printed all over the note, but those are the three main ways of separating the real ones from the fake.
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