What is "mains outlet tester" (for proper wiring) in the context of US power connector standard?
Here is a picture that shows what is tested with a USA outlet tester. Since about 1960, USA 120 V outlets have been required to have a third pin for earth ground. The neutral is also grounded but only at the distribution panel and at the service entrance.

This tester will not indicate a condition where the ground pin is connected to the neutral inside the outlet. I have seen an installation where all of the outlets were wired that way. The date of the installation was about 1962.

I believe that there was a time when outlet boxes were not required to be grounded in the USA. In that case, the cover-plate screw will not be grounded.



Here is a neon test light that can be used for two-prong receptacles. Note that the old extension cord is not polarized. I believe that there was a time when wall receptacles were not polarized in the USA.



Here is how a neon test light can be used to identify the hot slot when there is no ground available. The neon light draws only about 300 microamps with a solid connection and much less using a person's body as an antenna.