What are some best practices for evaluating ideas before building anything. You mentioned talking to users which is good advice but assumes that an industry has been identified already.
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.
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.
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.