Short answer:
You can't.
Long answer:
Well there are plenty of websites like http://www.ideacheck.io/ or ideawatch and so so many other. However they all have a common problem: They don't have people of your focus group.
This sites mostly get visited by entrepreneurs, developers and people who want to start a business. But not you customers. You should and have to decide by your own if your idea is good and what problems it should solve.
A big mistake people are making is trusting on other people opinions. If you do so, your app / product will grow into a huge mutated tentacle monster. Because everyone has a different opinion and everyone wants only features, that solves their paticular problem.
You have to analyze your market. What are the competitors. Are there any? How big is the market? Is it worth it? How big is the demand for your solution / product.
And then you have to identify the key features / problem solvers of you product. Keep it simple but good. Later you can always implement more features. Maybe talk with some people directly that are potential customers and ask them, what there needs are and think about how this needs would fit in your product.
Lean start:
A lean start is a common thing to do. If you don't want to go all-in. Make some concepts of your product, think about your USP and make a damn good landingpage. Sell the feature and your USP to the people. Newsletter signup for beta access or notifications if it starts.
You can gather a lot of data from a simple landingpage. First you get a feeling about how many people would be interested. Second you have a user base, if you start a beta. Which you should do! In the beta you can gather real feedback, from real users.
You can also figure out pricing this way. Even if you're product is not launched, you can make a pricing page, with different tiers. And track the clicks on the pricing models.
Sureley you should then build a modal or notificaton with: "Hey sorry, our product isn't launched yet, but you can sign up for beta".
This way, you get some feedback how many people would go the free tier, how many would go a paid tier.
Last, but not least
Just launch! Not matter what. People will always bitch about your idea. And no landing page can proof them wrong. Only the final great product.
I remember the start of dropbox. They posted a thread on HR and pitched their idea. Guess how many people were bitching about, that this is completly useless and why not use rsync?! etc.