One of the biggest problems with Argentina, in all of its regions from Mendoza to Buenos Aires to Salta to Rio Negro, is fake or counterfeit currency. So how do you prevent yourself from getting robbed (essentially)?
First, be vigilant. Most of the time you will get fake bills in Buenos Aires from taxis, kiosks and diarieros (news stands). But that doesn’t mean that a high end clothing shop or restaurant won’t pass you a fake bill as well. Or even a bank! So EVERY time you get change, especially of a hundred, look through at least some of the bigger bills for fakes.
Second, learn to spot the fake (called “trucho” in Spanish in Argentina) bills. There are some key points to look at. The watermark must be there when you hold it up to the light. The first part of this is the face of the “figure head” on the pesos. If the watermark looks drawn on or painted on, then its fake.
Also, look at the interlaced foil which is just off-center on 50 and 100 peso notes. It should be real foil, that you can see is a separate piece, interlaced into the note itself. If it’s drawn on or painted on, it’s trucho!
Last, the paper quality. This is not really noticeable if you are only holding one fake note. You have to compare it to a real one to feel the difference. Basically, fake notes are printed on computer printer paper and real ones are printed on, well, real paper. It is easy to tell when the notes are side by side.
My personal habit, whenever I am getting more than 20 pesos change is to hold the bills up to the light and look for the watermark. This is a common practice and I’m sure you will see people do it, now that you are informed.