I work with a lot of startups and serial entrepreneurs who use this method to discover what might be profitable:
It's like fishing, each website you put online is like having another hook in the water. You don't know which one is going to get the bites :D The key thing is don't invest too much time or money before you know you have any bites. Once an idea looks profitable, that's when you begin building your MVP. This way you're not investing time and money into a dead-end.