Not to sound too much the pessimist, but unless you are absolutely fluent and look porteño, there is a chance that at some point someone will try to slip you fraudulent money. This is not as large of a problem as in some other South American countries, but fraudulent bills do exist in Argentina. The down side of acquiring a fake is awkwardly asking for a different bill; the plus side is that it happens regularly enough that no one will be upset.
As you will be checking all of your change, expect that paying in any store will deliver an equally thorough check of your pesos.
The places that you should be most careful are taxis and clubs. Make sure to always check that the bills handed to you are real (often times these dark environments harbor secret evildoers.) If your cab driver does not turn on a light when handing you change (most do, expecting that you will check) ask for the light (la luz, por favor.)
Argentine pesos look and feel like play money. This is good, because it is very hard to duplicate the feeling of a real peso (as with dollars, fibers are placed in the paper to give it a distinctive feeling). When handling a fake, you will notice that it feels “wrong.”
Checking the bills takes about ten seconds. This is how to do it:
All bills have a water mark in the empty left part of the bill. If there is no water mark, it is not real.
•2’s, 5’s: Most people do not check the small bills. If you choose to, hold the bill to the light to find the water mark of the valiant war hero. The top of his head should end about 2 cm below the words on the other side, and his silhouette is 1 cm to the right of the red numbers. The bust is identical to the main picture on the bill. There are no embedded lines in 2 and 5 peso notes.
•10’s, 20’s: follow the same water mark procedure as with 2 and 5 peso notes. The second check on these medium-sized bills is the embedded line that runs along the left edge of the colored part of the note. It will be visible along with the water mark. Getting technical, it says BCRA $10 or BCRA $20 (Banco Central de la Republica Argentina)
•50’s, 100’s: Watermarks present, the difference with the big bills is the line that runs through them from top to bottom. Without holding the bill to light, you will see a broken line. When holding it to the light, it will connect fully, allowing you to read BCRA$50/$100.
A word on bigger notes: It is rare that you would ever be handed a 100, but be wary of 50’s. Most fraudulent 100’s come from street traders, which you should not use for a variety of reasons. I have heard the very rare story of getting a fake 100 from an ATM, but I’ve never known someone who was on the receiving end.
That said most people don’t have this problem, but it does happen. Always better safe than sorry!