You probably can't. If you've got an established relationship with somebody, that relationship is virtually guaranteed to affect the answer. Usually, they'll lean too far to the positive side ("yeah, that's a great idea!") as you mentioned, but it goes the other way sometimes too. (When I told my dad I was starting a business, he was so worried for me that he practically became an instant pessimist on anything I said about it.)
The best way to get honest feedback is to take it to the customers, or take it to the potential customers. The specific channels that might be available to you, or the specific actions and tasks that you'd have to do will vary depending on what you're building, but these are the people you need to talk to, not your friends. These people have no relationship with you previously; for them it boils down to whether the product is the right thing for them or not, and it doesn't get much more honest than that. So focus on these people and avoid the people you already have a relationship with.
Lastly, I wanted to say something about the last thing you said. You actually want to be getting feedback before your startup launches. The sooner the better, in most cases. Don't wait until you've already put tons of time and/or money into the idea or you may discover that you've built the wrong thing. The sooner you start interacting with potential customers, the sooner you'll find out if you're on the right track or need to pivot.