There are so many variables, one piece tools, tools with moving parts, tools with motors, tools used to measure, levels, cutting tools, machining tools.... etc.
A simple square only needs to be square and the test is very easy. Register it to a line or edge, make a line, flip it, make another line close to the first, compare the results. The lines should be parallel.
How parallel? Close enough you that can't see any difference.
Measuring pieces that have to fit together requires more precision, but that all depends whether they are wood and will be glued OR metal pieces that need to rotate. The tool must fit the application. A nice $100 Starrett square will be a family heirloom to be handed down, while a $10 Irwin won't be. You would be a lot more careful with the Starrett, than the Irwin and for good reason. You wouldn't want to drop either one, but especially the Starrett.