AC adapter power supply design/component quality
You can compare the approval file numbers with those listed on (say) UL's website. If there is no file number on the device it may be dangerous. If the file number is there, and matches a plausible manufacturer, however, it may still be counterfeit.

For safety you could test with a HiPot tester for isolation between mains and output. This can be destructive (to the adapter), if the test fails. 

There is no way to tell from the outside if the device is reliable or might put damaging (to whatever is connected) voltage at the output. It's less likely that it would be dangerous to humans etc. if it passes the HiPot, but there are still ways of making adapters dangerous and pass- for example conductive debris rattling around inside or the use of substandard and unapproved Y capacitors (the latter bridges the mains and output so is part of the isolation barrier). 

Efficiency is quite easy to measure- if you have the right equipment. Measure the true power input with various (and no) loads attached. The load can be a simple power resistor but it's easier to use an electronic load (and you can automate testing at various loads). You probably also want to know the output ripple and load regulation, which can be done with a multi-meter, and/or oscilloscope or spectrum analyzer if you need to know more information). With an environmental chamber you could test the regulation and efficiency (and functionality) at various temperatures. 

You could even get some idea of expected lifetime by doing accelerated life testing at high temperatures.