I don't know if you've noticed, but increasingly, when I look through potential magazines for my article pitches, I see there seems to be an alarming number of such magazines who only seem to use their own staff writers with minimal or no scope for free lance contributions. So where does that leave the freelance?
Is it still worth pitching to them on the off chance that they might take you up on it? Or do you leave yourself wide open to, at best being ignored completely or getting a polite decline, or, at worst, see your article idea written by a staff writer in the magazine you pitched it to!
Now, there's no way of proving that they 'stole' your article. There is no copyright in ideas and it could be the reason you didn't hear back from the editor was because they'd already got the topic you pitched covered. The only reasonable and professional thing you can do in the case of a magazine that has the staff writers' names on all of their articles is to move on and find another magazine that welcomes freelance contributions.
You could also save yourself and the editor of such a magazine a lot of wasted time by actually contacting them first - by phone is ideal, if you can get a number, as it's not so easy to avoid a phone call as it is to ignore an e-mail - and ask them if they are open to freelance submissions. Then you'll have your answer and know either not to bother or to pitch.
It's always worth approaching the editors with your intention to pitch article ideas as you never know if they'll say yes or no. Maybe a staff writer is off and they need someone to cover, or perhaps content is a little light for a couple of issues and you can help them out. Maybe you are an expert in something topical and their staff writers aren't and the piece would be better written by you. Who knows? So don't be afraid to try but do move on if it's an outright no. Remember to keep checking periodically as editor's change, staff writers leave and there may be openings in the future.