3 stages:
1. Get as much feedback as possible (don't say: "what do you think?", say: "my friend has this idea, he wants me to join. I think it's not such a good idea. What do you think?" - this way people will be more willing to tell you the truth about the idea (if they know it's your idea, they will be afraid to offend you).
2. Refine your idea (and the solution / value it provides).
3. Validate the idea ( see how on this answer thread in Clarity: https://clarity.fm/questions/596/how-do-you-get-access-to-the-right-people-to-validate-an-idea )
I've successfully helped over 100 startups. I'm happy to provide further assistance.
Best of luck