Discerning important work from irrelevant is vital if you want to create (or achieve) something that matters.
Important work is always hard and unpleasant, it requires amazing dedication, an awful lot of energy, it isn’t measurable and its outcome is (most of the times) uncertain.
Important work is the work you don’t want to do, the work you’re constantly avoiding. It’s the work you know you have to do, but you’re scared to do it because it’ll expose you, because it’ll open you up for failure. But that’s a good thing, because you can’t really do anything important unless you’re prepared to fail.
Irrelevant work, on the other hand, is always easy, pleasant and has a pretty certain outcome. You don’t have to fear failure if you’re doing irrelevant work.
Hence, I believe that the easiest way to know for certain that something is important is to ask yourself if you’re avoiding it, if you’re feeling a certain aversion towards it. If the answer is positive you should probably drop everything you’re doing right now and start working on it.
Alternative, of course, is doing work that doesn’t matter.