If you run into an employement contract that includes a "prior inventions" disclosure, look it over very carefully. If it indeed protects you, use it!. Disclose, in broad language, ideas that you want to protect. Be only as specific as you need to be to avoid ambiguity.

If your employement contract doesn't include such a disclosure, negotiate one in.

To establish that an idea is yours, it might help (depending on the laws in your area) to keep a private "lab notebook" for personal work. But be very, very careful not to mix in non-private work. I rather suspect that the guy who ran into problems with someone else claiming his thinking had no documentation on his prior thinking.