I imagine any country where taxation isn't roaming gangs, has a public description of how the tax system works, allowing you to calculate. But as you suggest, it is often hard to decipher. Canada's is supposedly relatively simple. If you request a paper kit, the whole thing is given as a step-by-step algorithm with lots of tables.

I did my taxes by hand until quite recently and it strained my working memory to the limit. Too many things to keep track of. Most people just copy over the requested fields and pray, I think.