There is no single, universal rule for selecting a high-quality power supply. Nevertheless, various indicators provide circumstantial evidence of PSU quality, and some guidelines are generally helpful.
First, always buy a power supply from a reputable manufacturer, and look for reviews of it before you buy. Avoid cheap, generic power supplies, which tend to be substandard. Look for reputable brands that offer solid warranties and support. Corsair, Raidmax, Seasonic, and Antec are three manufacturers with reputations for producing high-quality power supplies, though even they may offer a few duds among all the studs. Do your homework!
Larger, heavier units are preferable to puny, lightweight models. Higher-quality power supplies almost always use bigger and better capacitors, chokes, and other internal components, and they come outfitted with larger heat sinks for superior heat dissipation all of which translates into more weight. Larger cooling fans, which typically move more air while making less noise than smaller fans, are another plus.