Identify Fake Money x Free R€D

€€€ COUNTERFEIT BANKNOTE REVELER for Free €€€. When somebody give me 50 € or 100 €, suddenly I start to sweat, asking myself "It's a fake? It's a counterfeit banknote? ". I really don't know how to recognize real or fake cash. For me 50 € are a lot of money and loosing them could be a really bad deal.