This is my line, so I can give genuine advice here:
If you take dollars, it's very easy to make sure they're genuine, and don't feel embarrassed to check the notes in front of the client. Firstly, don't take a dollar note that is dated before 2006. (The date is on the left hand side, about two thirds of the way down the note). Secondly, (and this also works for the NEW R200 note)if you look at the bottom right hand corner you will see the denomination of the note printed in numbers. If you look at the number with the note facing you directly, and then tilt the note away from you, the numbers will change colour. Some notes change from gold to black, some from green to black, some from black to green.... If the number changes colour, the not IS genuine.
Forgers cannot get access to that ink and the process of printing and treating that ink is currently impossible for a non governmental agency to reproduce.
So if you have say, 6 crisp $100 notes, it takes a matter of under 10 seconds to look at each one, tilt it slightly and see if the colour changes.
Here's a link,
http://
www.indigoimage.com/count/feat3.html#ovi
That explains the Optically Variable Ink.