The safest way to avoid fake parts in your product is not to buy jelly bean parts.
Every time I need a chip, I will search mfgs' website and find the most appropriate one, not the most commonly used one.
If a chip is neither ridiculously expensive (i.e. LTZ1000A) nor used in massive quantity (i.e. FT232), no one will clone it.
So far, I never encountered a single fake CP2104. I heard of some very rare fake CP2102, but the smaller packaged CP2104 is still fine. Also, the cheap price of CP210x makes it not favorable to buy fake chips, so no market, no producer.