Simple answers depending on what your product is:

1. Post the product to eBay and test pricing
2. Post it to Craigslist
3. Ask all your friends and family to buy it
4. Take 20 people out to lunch that are in the target market, tell them that in three weeks you are planning to launch a business that sells x. Ask them a lot of questions about their needs as a target customer - but do not sell them at all. At the end, ask if they know anyone who might be a good fit for your product.
5. Create landing pages and test them with Google Adwords, Bing Ads, or Facebook ads. Get the sale but do not charge the credit card. This is relatively low cost, gets fast results, and is my specialty - I'd be happy to help.

Cheers -
Nick