Like everybody else dealing with large amount of cash. There are quick ways to check by eye and touch [https://www.wikihow.com/Detect-Counterfeit-US-Money](https://www.wikihow.com/Detect-Counterfeit-US-Money)
or special “pen markers” and lamps/machines to check the ink and paper is correct.
Edit: right now manufacturing modern US dollars it’s an extremely difficult technology to pull off. The source materials and the process it’s of course extremely tight.
Probably out of reach for criminal cartels, but not impossible to attempt for an “adversary nation” to destabilize the economy.
There is a historic precedent in ww2 Germany tried (unsuccessful) to flood Britain with fake Pounds notes
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