By understanding what the product idea is as opposed to how it's implemented.
It's amazing how many people think you can't test an idea without tons of effort. Say you had an idea of selling shoes online. You need an IT team to make a web site, a manufacturer, and a distribution center right?
Well yes you do. But the website can be pictures of shoes and a form that sends you an email. The manufacturer can just be the local shoe store. The distribution center can be your kitchen table and a few postage stamps.
How is that a test? Well if the problem is that no one is ordering shoes you have a result. If the problem is that you're spending to much time in your kitchen mailing out shoes you have a result.
This test proves whether people will buy shoes online. It gives you the kind of data people who might invest in your business care about.