It's actually very simple. Hard to do but simple. There is no sophisticated insight or knowledge or process you need to know. It's simple but hard.
Suppose you have an idea for some software for travel agents. Before opening an idea or writing a single line of html you go to the phone book and find the number of a travel agent. You call them and say :
"Hi can I speak to the manager please. Hi there, my name is Sam and I have an idea for a computer software tool that might be useful for Travel agents, I'm not selling anything but would you have 15 minutes available for me to talk to you and get your feedback ?"
Do that 20 times. Talk to 10 different people. List any repeated phrases, objections or suggestions. Ask each of them "Do you think this would be useful to other travel agents ?".
Now if you have the guts to do this very simple but difficult thing enough times that you actually talk to 10 different potential users you will know whether there is a market and if there is you will already have real beta testers lined up.
People may want to pick holes in this "what about..." "what if...". Yeah, you can analyse on the internet all you want but the fact remains, if you do this the chances of you building something people will buy goes up dramatically.
And the reason people don't do this is almost always fear.
