There are 2 ways:
1. Talk to individuals who you expect to be your users.
Ask them how they currently do what your idea makes easier to do. Try to find out if they have any problems with it, but don't push too hard. Just listen. If they seem at all receptive, tell them about your idea. Then pay attention to everything they talk about (good or bad). If they don't seem interested at all, move on. Find the next person to talk to. Your aim at this stage is to find people who do think your idea has some value. The uninterested ones provide data to act on in case you don't find anyone interested in your idea at all.
If you can find even 1 or 2 people who love your idea and really get excited about some feature, then build that as quickly as you can and get them to use it.
2. Just build whatever little thing is useful to you immediately. It's ok if it's ugly and incomplete. It should just give you more of at least one of the following: time, money, fun, happiness.
Then show it to other people who you think might like it. Most reactions on design you can safely ignore. People have a natural tendency to talk about how things look because it's what they see first and feel obliged to say something. It happens on HN all the time.
What you want to look for is things people really love or hate about what the thing does. Also look at the data on what people are actually doing with your product. They often say one thing, and do something completely different. So pay attention to actual behaviour.
Out of all of this, you might notice one or two things really stand out. When they do, zoom in and focus on them to an extent that will seem ridiculous. It should make people say things like "This is just a feature, not a product". Then you know you're really onto something.
The more interesting and novel your idea, you are better off going with route 2. The new conventional wisdom (of Lean) suggests you should mostly always go with 1. But if you look at the landscape, the best ideas come from 2. The chance of failing may also be higher with 2, but that's where the artist in you comes out and pushes the boundaries.
Talking to people about your idea too early may kill the spark of magic that brought it to life. Make it real asap.
These gentlemen say it better than I can - http://www.zefrank.com/theshow/replay/?p=363
http://www.thisislondon.co.uk/lifestyle/london-life/sir-jona...