Depends on what kind of hardware though. With certain things, you can test a product market fit with some ducttape and a lot of creativity. There's a pretty good book on that called "One simple idea" by Stephen Key, which, whilst it has a focus on licensing such producs out, should come in handy for stress testing a hardware idea. And for the more high tech folks that won't go to the mall to test out whether their stuff will sell, there's still Kickstarter.