No one can be sure your idea will sell until it's actually sitting on--or flying off--the store shelves. But you can get an idea whether your idea has potential at an invention show when visitors to the show: 1) flock to your booth, 2) love your idea or 3) wonder where they can buy your product. People who attend invention shows see a host of inventions, so you can learn a great deal about your product by seeing how attendees respond to your and others' ideas.
First, pay attention to how many people come to your booth compared to the booths around you. People will go first to booths where they see a product that fits their needs or is substantially different from other products on the market. If people aren't coming to your booth, it could indicate that your product isn't perceived as innovative, your product isn't addressing a need, or people don't see how your product will benefit them.
Invention shows not only attract industry marketers but also a wide variety of consumers. You'll generate more feedback if you have a product that a sizable percentage of the general market will buy than if your product is aimed at a limited market or is solely intended for industrial use. You just won't see enough potential buyers of the product. Though the invention shows, particularly INPEX, run extensive publicity programs to aid inventors with products for specific markets, they still rarely attract enough interested parties for inventions targeted at a limited number of buyers.