Here's a simple process to follow:

1) Identify a pain-point you are personally experiencing in some X-industry

2) Search through to the deepest darkest hole of the internet to find an existing solution

3) Identify whether current players are poorly marketing themselves OR if the market is too small (only 500 people need this??)

4) If after analyzing 3), the idea is viable, go for it, else begin from 1) again.