Make something and get someone to buy it. I worked for quite a while on a side project but always had doubt until the first sale. People giving you money for something you made is a big confidence boost.

Teach. I've taught a college course for a semester and was the main trainer/teacher on a product/api we developed. Teaching others something you know will make you aware of how much you do know and how well you know it. And it is enjoyable if you don't mind the public speaking part.