It has happened to me at least twice in Peru, that someone gave me a fake 5 soles coin. Most shop vendors were able to spot this instantly and refused to accept it.
How can I recognize a fake/false Peruvian coin?
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I was initially sceptical that this was a common problem as I didn't encounter it the two times I went there. Then I did some research and you're definitely not alone.
There are a few blogs on this matter that have tried to give their thoughts on the subject.
According to this video (in Spanish, may be the automatic subtitles could help) distributed by the peruvian governtment, these are the main steps to detect fake coins:
You only speak about coins, but there is also a video available in order to help to detect fake notes: