Most likely it is done with TL15 technowizardry. Every note is its own encrypted computer system and will tell if it is genuine in about half a dozen ways if prompted the right way.
Also, larger sums would exist mostly as electronic payments, and the amount of actual notes would be almost neglegible in comparison to the actual amount of money.
Oh, and then there is the thing that forging imperial credits would be an imperial crime (and one the Imperium would come down hard on), and people could not outrun their crimes as easy as with some other crimes.
Just throwing out ideas...