If you have an agent or manager? They could gently bring that up in a meeting but unless you have a real track record of production? I'd be worried that they'll never want to have a meeting with you again.Yeah I've heard that before. No way to go around it huh ?
If it were me? I'd keep a very detailed LOG of who I query and how. Whether or not you had to sign a release. Who you dealt with. How long it took for them to respond. You never want to try and remember any of this later on down the line. Plus? You keep all emails if you're querying by email. Always copyright your scripts as well... Not just the WGA's time stamp.
But also know and realize... Only execution can be copyrighted. You can of course, type up a treatment or even a smaller synopsis and copyright that as well. Up to you. The only real way you can fight anyone in the industry legally is if you have a copyright on your material. Unfortunately, anyone can take the concept and kick it up a few notches if they don't think your logline or screenplay gets the job done.
That's ultimately why it's better to have your material submitted via an agent or manager... The producer is always going to think twice before stealing a concept if an agent or manager is involved.