I am 99.99% sure that you would go and build a prototype ( can be a paper sketch) before you actually go and build a real product.
Here is a good piece of article which covers some tools which can help you create nice little prototypes. Most of them are paid but comes with trial versions. So be quick to create your prototype. Even if it leaves a watermark in there you should be fine explaining your idea ( unless you know that your audience wont like it and you might loose a big deal)
