I have what is probably an unpopular response to this question, which is that your own passion for that thing matters.
You do the small thing, and maybe you get some signs it will do well, but it’s also important that you get signs that you can persevere to make a bigger thing.
Maybe it even went badly, but you lost track of time doing it. You found yourself tinkering, and you’re sure you could spend many more hours building something more complex.
Remember, the tinkering is important to innovation, and in my experience, the way you feel doing that thing transfers to your audience, which makes that thing spread. (I called this “The Pump” in The Heart to Start).
The external signals should be very clear. The external signals can be a Black Swan in themselves. For example, when I got my first book deal, I had a good external signal that my idea was worth pursuing: I wrote a blog post, and it went so well, a publisher reached out to me. That’s a super clear external signal!
But that alone wasn’t enough for me to decide it’s worthwhile. I had to also look internally and ask myself if I enjoyed all of the activities associated with writing enough to live through six months of writing a book.
I’m tempted to say that you get super clear external signals, but you have to also listen to your internal signals.
However, perseverance is important. I should have added to this list of things that you also should seek out rejection and failure, because those mean you’re stretching yourself. For example, Seth Godin has rejected my requests several times leading up to him endorsing my book. Even though I don’t enjoy rejection and failure, I have learned to push myself toward it—as long as it’s productive rejection/failure, in which I can look back and say I learned something.
Not the clearest answer, but that’s what I’ve got right now. Something more for me to think about.