Similar to Rule 34 of the Internet, if your startup problem exists, there is a sub-reddit for it.

Go to Reddit.com Search and look for your target market. If you're doing daily deals for campers for example you could go to //www.reddit.com/r/camping.

Post an honest submission to your sub-reddit of choice, ask them what they think of your service and whether they would sign up. Ask for feedback and engage with them in the comments section of your submission.