Hold the bill up to the light and check the strip, it'll say what denomination it's for. There's also a watermark of the presidents head visible when held up to the light. Check the ink in the corners noting the denomination to see if it has metallic flakes in it and finally look for the red and blue fibers throughout the bill.
TIL
I know people that have pulled those out.
They'll still have the watermark and metallicized ink to contend with assuming they bleach a lower denomination bill while retaining the security fibers.
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