How to know if a product is useless or just needs more promotion?

I have so many SaaS ideas, and I just want to build them in my free time. I know I should find/talk to users first, etc, but if anything, it's good to practice coding skills.

Anyways, I'm not sure when I should move on to a next project or keep promoting what I currently have. Submitting to directories and making social media posts takes time, and obviously a useless product will have zero users regardless of promotion.