It's tough because people are nice, but as an entrepreneur, I learn the most from the negative advice I have gotten from mentors (I've been ripped apart many times).
So now, when I give advice, I give my true honest opinion if I don't think their idea will work. I just preface with it saying, look, this is only one opinion, and not to take it personally.
And when I get ripped into, I always listen and ask questions, but I don't necessarily let it discourage me.
I think you can always tell by people's expressions if they are holding back. If you think they are, you should ask them: "It seems like you have some reservations about this, what are some of the risks you see?"
With that, you'll be able to get more insight, and you just have to remember that there's tons of risk, it's just up to you to decide how risky each situation is (and how to mitigate that risk).