Internet reviews and tests. Consumer magazines tests. And also reviews by a major retail chain that I trust (namely, the FNAC, for the couple French posters here).
I check reviews from wherever I can get them. I pay particular attention to the bad reviews. If they repeat the same problem and it’s a major issue, I suspect bad quality. I researched one item recently that got bad reviews for being hard to put together, but was otherwise positive.
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Research, reviews and return policy 
Amazon reviews are no longer reliable at all, particularly for electronics, so I use fakespot to make sure the reviews aren’t totally phony.