How to test DC adapter for reliability and performance?

[Gyro]

Well assuming that little load can sustain 24W continuously, then it will tell you that the adapter can sustain its rated output. It won't tell you anything about its safety though.

[Zero999]

You need a tester which applies a high voltage between the primary and secondary sides of the power supply.

A visual inspection is also required, as a high voltage test won't pick up silly design faults such as parts not mechanically secured properly.

Ideally the manufacture should have designed it properly and tested it, if necessary so none of this is required, but it's a good idea to test and inspect a sample of units.