Most of the time you won't know the person you're talking to that well, and unless you have a conversation beforehand - how do you know what their ethics are?
I was assuming the context was signing NDAs as a condition of employment. Even if that's not the case, I'm still going to talk to someone a little before I just spill the beans.
Company interviews are quite different then just sharing an idea with someone. If you're going to a company to be hired and paid, they aren't going to likely be telling you ideas - more of a general idea of what you might be working on, and to see if you're a fit. You'll then be made an offer for the job (or not) and will have to sign papers, including an NDA to accept the job, likely also copyright-related agreements, etc..
Some companies don't require that their employees sign NDAs about the work they do. These companies still manage to keep confidential information confidential because in the interview process they are able to weed out the people that lack integrity.
So per my original comment, there are lots of indicators of a lack of integrity. Just ask enough questions about someone's life, their values, their ambitions, how they'd respond to various conflict situations, ask them their opinions on just about any issue. If you get unbalanced, dramatic, polarized results, it indicates a lack of integrity, which might crumble altogether given enough incentive, monetary or otherwise. If this approach is too sketchy, or you don't want to miss out on otherwise talented people, use an NDA.
I know that might sound a bit bullshit. I guess this is speaking as someone who lacked integrity in the past that feels they can generally recognize it with some precision in others now.
Those aren't the type of people you need to worry about though - it would be the sly ones, and they are hard to know who they are. They are friendly until they don't care to be. I'm unsure if people purposely do this or they just become inspired by the ideas, feel they could do them on their own, and then start - I'm sure there's a mix, though you'll get a mix of honest and dishonest people in the both groups, the ones who are willing to sign the NDA, and those who aren't willing to.
