I am reminded that you mustn't put too much stock in what any one person says--if people like your song, you will know(as the other way)--

It is best to try a new song out on a group of people, never only one--the principle being that, though a popular song may sell a million copies, there are more than 200 million folks in the US(for example), and the more you play for, the more likely you are to find someone who likes it --

--I think nothing written above will help you as much as simply reading and studying the songs (and poetry) that speak to you--and, to repeat, there is no test of a song as good as in performing it for an audience--

Also, trust your own feelings about your work above others, no matter how learned and venerable they may seem--

You said something about being afraid others might borrow your ideas--you shouldn't worry too much about this, first because each writer will create something different from the same idea, and second because most people have so little imagination that they will fail to see potential of a good idea anyway--