Excellent question. Already says a lot that you're asking it in the first place.
For starters, I wouldn't ask for idea feedback from anyone aside from your market. Period.
Every expert will have an opinion, every family member will weigh in, all your friends will find an issue (or worse give you false reassurance).
The only reliable way to get feedback on an idea is from the target market itself.
If you haven't read Lean Startup (https://www.amazon.com/Lean-Startup-Entrepreneurs-Continuous-Innovation/dp/0307887898), that is the first thing I would do. If you're in a hurry, google around and get the gist of the method and you should be fine.
Then, I'd find a way to describe your idea in a sentence. You can be as vague as, "I'm creating a service that delivers groceries to rural areas." No need to get caught up in the specifics. Just be able to articulate the crux of the idea clearly. From there, you iterate.
If you can get a prototype in front of your market, that is the best way. If you can't there are plenty of ways to test the idea that don't require a ton of money and time - for example, surveys.
Here's a quick primer on how to do that in a way that will actually be useful: http://www.thatseemsimportant.com/2016/08/04/customer-survey/
Hope this helps!