As locusts says I wouldn't worry too much about that - I think the key is to not discuss the idea too much online, people are always going to come up with similar ideas and if you've posted your idea around online you are always going to end up wondering, which will do your head in.
With something like a film script you are circulating on spec then people do advise you post yourself a copy of the script in a big strong envelope and then leave it in a draw. If the worst happens then you can at least show you had the script in a certain form at a certain date.
Actually it may prove more useful to cover your ass if someone else thinks you stole their idea. There are lots of cases of "things that make you go hmmmmmm" like Firekind/Avatar, A love Like Blood/Underworld, The Walking Dead/28 Days Later, etc. which are unprovable but do make you wonder (the last one is tricky as development seems to have been in parallel and the opening sequence is actually from Day of the Triffids, although I don't think Kirkman has ever acknowledged it, as the 28dl chaps have) but it can get into the courts, like
Sophia Stewart suing because she thought both The Terminator and The Matrix ripped her off (ironically Harlan Ellison successfully sued the Terminator folks and Grant Morrison has claimed they had copies of The Invisibles on set when shooting the Matrix) and there was also
a lawsuit over League of Extraordinary Gentlemen that pissed Alan Moore right off. Of course, if you have a success (or LXG) people will be all over it to get a chunk of the cash, so watch your back.