There are several ways to determine this. First, check out customer reviews, which will soon reveal the flaws in a so-called "high quality" unit if there are any (though keep in mind that the occasional manufacturing error isn't a reflection of quality overall). You can also check to see if a system is NSF certified. It's common for water softeners to be NSF certified for quality of build, and an NSF certification can give the best third-party assurance on this. Finally, durable materials and resin indicate a high quality build. Avoid plastic parts, especially fittings and the bypass valve, as much as you can.