It takes understanding your own impulses and cognitive biases.

https://en.wikipedia.org/wiki/List_of_cognitive_biases

Most people start a small business for tax reasons, and the few lucky loud clowns claiming hyper-scaling BS is sustainable simply disappear within 5 years.

Best advice, answer the simple question "will random people buy this when no one is looking?"... if you have to explain/sell a product/service more than 5 seconds, than that business model likely isn't sustainable.

I also see a lot of smart folks that think they will get paid for excuses... only lawyers, PR, and politicians are in that business. =3