李鬼

From the classic novel Water Margin (《水浒传》 Shuǐhǔ Zhuàn): the bandit Li Gui (李鬼 Lǐ Guǐ) ambushes travellers by claiming to be the famous outlaw 'Black Whirlwind' Li Kui (李逵 Lǐ Kuí). Unfortunately for him, he picks the real Li Kui as his next mark and is promptly killed. Today 李鬼 is the standard label for a counterfeit goods or a copycat business — and 真假李逵 (zhēn-jiǎ Lǐ Kuí, 'real vs fake Li Kui') is a stock phrase for telling originals from fakes.

Don't mix up 李鬼 (Lǐ Guǐ, the impostor) with 李逵 (Lǐ Kuí, the genuine Black Whirlwind hero). The character 鬼 (guǐ, ghost/devil) is the giveaway — that's the fraudster.