The first thing I ask a founder is how often does the target customer have the problem they are looking to solve and how much money are they willing to spend to solve it. The more often the customer has your problem and the more money they are willing to spend - the easier it will be (all other things being held equal) to get customers and iterate. Sequoia calls these problems hair on fire problems. The second thing I ask is who else is trying to solve the same problem and what do you know that they don't.

This is great advice and advice I wish I taken onboard earlier. As a technical founder Ive found myself making a large set of assumptions which are usually proved to be incorrect when going to the market.

Going into the market and talking to your customers early on so you can answer those two questions will really help tell you if you have a viable idea.

>>> assumes that an industry has been identified already.

This is one of the key take aways from customer discovery - is your customer actively looking for a solution to the pain / problem you've identified. If your customer is actively looking for a solution than it is easy(ier) to identify and talk with them. If no one is looking for a solution then it may be that you haven't identified a real pain, you haven't identified the correct customer, or you are to early.

It's easy to say that you can't talk to customers because the industry hasn't been identified, but that is a bad statement and should be avoided.

Ok, so following what I think you're saying, how does one find people with problems they're looking to solve?

Ideally you'd be a user of your own product, someone who is actively experiencing the pain and is looking for a solution. Then you'd look for people like yourself.

Otherwise you still want to follow the general pattern of talk to whomever stands to gain the most from your service. Note that the person that stands to gain the most might not be who you first assume it is.