You're right. I meant a side project that you build for profit.
I totally didn't imply it's bad to build something for your own use without making profits of it :)
How would you formulate this question?
In that case, I wouldn't call it a side project at all. Whether it is your full-time job, or something you are building as a part-time business, the validation of the demand doesn't change -- Make a prototype and ask people if they'd pay for it. If they say yes, move on to an MVP. If not, ask for feedback and either try an updated prototype or try something else.