In theory, you can get them to sign NDAs, to say that they won’t tell people and won’t copy your idea, and to make sure that it is clear they have seen your idea.
If it is a big company you’re talking with though, and you’re a small company / random guy with ideas, in practice it’d be surprising if they’d worry too much about breaking them (and quite a few big companies will refuse to sign your NDA), but at least you have evidence that they saw the idea.
If by any chance it is software, in my experience of software development, 99% of the work is on the development of an idea, loads of people have ideas that sound neat to them but not the skills to make it, and 99 times out of 100, the idea turns out to be stupid once someone who could build one starts thinking about building it. I’ve quite a few times had people saying “I’ve got a great idea for a program/website/gadget/whatever” that they obviously think is the greatest thing since sliced bread. Inevitably they want to give you a share of the supposed profits, rather than actually pay money for it to be developed.
Joe