I think you simply have the same faith in them that you have in anything else you buy: real pharmaceuticals, food, hygiene products like anti-perspirant or shaving cream,..
Though I tend to avoid many products made in China, I'm 1) realistic enough to know it's pretty much impossible to avoid them all; 2) aware that there are some very high-quality products that are made in China; 3) aware my rationale for avoiding products from China is as much tied to politics and human rights as it is to product quality; and 4) of the belief that some products pose a greater chance of risk than others.
An ingredient that a Chinese supplier provides to, say, an American or Japanese manufacturer probably receives more rigorous acceptance testing than the same material going to some unnamed company which will stamp the finished product "Manufactured for SuperMegaStore". For smarter or dumber, I'm more careful about country of origin for products I take internally than I am for products which I use outside my body.
There also are counterfeits of some products; the higher the margin, the more likely someone will go to the trouble of counterfeiting it. I might find a lower price for a product on ebay or Amazon, being sold by some lone-wolf seller, but I think the risk of not getting what I think I'm purchasing is higher there than it is at Costco or my local pharmacy.
I don't mean to pick on China alone on this, either. There are other countries which manufacture products I would pass by on most of the same criteria. China simply is the big fish in the pond and has a long documented record of not offering what was advertised.