It depends. For home improvement projects, you can see what tools people are using. If a home repair channel is trustworthy, that may be a good start. General market research can help as well. For instance, post-China-buyout Craftsman should be avoided. For nearly anything else, it really depends. Name-brand electronics are usually safe unless they're small items (eg, power adapters, USB cords. Batteries are nearly impossible to purchase online without getting ripped off.) For clothing, I would just generally recommend only buying from thrift stores. For a lot of physical items (eg: door knobs, fuel hoses, etc.) it is actually quite difficult to purchase online. In person, we can tell a lot about quality just by touching the object with your hands. (eg: a flimsy shelf is not self-evident online, but is obvious in the hardware store.)

I suppose that's a long-winded way of saying that nearly every category of item requires its own strategy. For a brief period consumers were winning the information war and you could just go to Amazon, read the reviews, and get a superior product for cheap. We're now in a modern-but-old-fashioned situation. It's quite difficult to know if you're going to get ripped off, and you're forced to rely on more blunt heuristics. (eg: trust specific brands, buy things in person, etc.) None of these are perfect, but they are quickly becoming the best of some bad options.