Examine the trademarks, certifications, and warning labels. If they are very clear and distinct, it's likely genuine. Also, counterfeit products usually don't have custom-made stickers; this is often a result of files not being properly processed during mass production, which is then corrected in subsequent batches. I also have an English label with a battery here, and I've taken one apart before. The origin is uncertain, but it confirms that it's genuine.
Finally, I want to say that the profit margin for K5/K6 is already quite low, and there are high risks associated with counterfeiting. Just like pirated "Mei Xue Bing Cheng" (Snowy Ice), it's not worth it.
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