I think it depends on the audience. In my case, I built my side project to try and solve a real problem that my accountant had. It worked, and several other businesses — mostly accountancy firms but certainly not exclusively — have paid for premium subscriptions. Some businesses have been paying me every month for years.
I think my audience is the type that prefers it if you pick up the phone and have a conversation with them, or email them, or write them a letter. So that’s what I did. I had lots of conversations with lots of people. I tried to build relationships to make sales, and the validation part only came when the customer actually paid money.
I tried the build almost nothing and throw up a fake credit card form like some people suggest, but it didn’t work. I know it works for some people, but it didn’t work for me.