Keywords, Google keyword tool, forum chats, Reddit comments.

Use tools like these to find EVIDENCE that people are trying to find a solution (similar to yours) to the problem they have.

If you find this evidence, it proves that:

1. There is an inefficiency present (opportunity is simply inefficiency by another name).

2. It shows that the problem is painful enough that people are trying to find a way to solve it. And..

3. It proves that the customer is aware enough of the problem to be able to see you as a likely solution (so you don't have to educate them).

I used this process to come up with my own successful ideas. One of which being: https://myapptemplates.com.

This video will help shed more light on the above https://www.youtube.com/watch?v=exMoRoaxKtQ