But while we're familiar with our own currency, would you know how to check your change abroad on holiday?
See also: How the value of the pound affects you
It's a real risk, with an estimated $70million worth of fake bills circulating in the US and 353,000 counterfeit euro banknotes were withdrawn from circulation in the second half of 2016 alone.
The good news is that there are simple ways to check if a note's real or fake – this is the official advice.
How to spot a fake euro
Euro and US dollar notes
According to the European Central Bank, 80% of the fake bills it recovered last year were either €20 or €50 notes.
A new €50 note was launched in April to try and combat the counterfeiters, but overall the advice remains the same.
"Genuine banknotes can be recognised using the simple 'feel, look and tilt' method," the ECB explains.
This is how it works:
Feel – The paper euros are printed on should feel "crisp and firm". The notes also have raised areas – on the front there will be a series of short raised lines on left and right edges of a note, the main image, the lettering and the large "value numeral" should also feel thicker than the rest of the note.
Look - Look at the banknote against the light. The security thread appears as a dark line. The € symbol and the value of the banknote can be seen in tiny white lettering in that stripe. Higher value notes also have a watermark – which become visible if you look at the banknote against the light or becomes darker if you put the banknote on a dark surface.
Tilt – If you tilt the note against the light, there are two key changes that happen. First, the shiny number in the bottom left corner displays an effect of the light that moves up and down. The number also changes colour from emerald green to deep blue. Second, The hologram – the silvery stripe on the right of the note – reveals a portrait of Europa (a figure from Greek mythology) as well as the € symbol, a window and the value of the banknote.
Full details can be seen here for all the euro notes currently in circulation.
How to spot a fake dollar bill
Possessing or or using counterfeit dollar bills carries a maximum sentence of 20 years in prison in the US, so seriously do they take their currency – but that hasn't stopped people trying.
Inside the USA, the most common fake note is the $20 bill, while worldwide the $100 is the most frequently counterfeited.
Different denominations have different levels of security features on them – but all feature raised print that you can feel.
That, along with crisp printing and the 'feel' of the cotton-based paper used to print all US notes are the only official security features on the dollar bill – which has been unchanged since 1963.
However, there is also a trick with a magnet that can help you authenticate a real one too. If you put a slight crease in the middle of the dollar bill, then hold a magnet over the number "1" in either of the bottom corners, the bill should move.
With higher denomination bills things are a little simpler. $5, $10, $20, $50 and $100 bills all include an embedded security thread that shows up under a UV light as well as a watermark that's visible if you hold them up to the light.
The most recent notes from $10 upwards also have a colour-shifting numeral in the lower-right corner of the note.
You can see detailed information on the security features on all US bills here.