I know I'm polyamorous because even though i have tried very hard not to (out of a sense of loyalty to my wife, not knowing at the time that there was an ethical alternative to cheating, and I did not want to be a cheater), I keep falling for other women, and even though I have kept those feelings bottled up and have not acted on them, they do not simply go away.
I know I am polyamorous because I have grown bitter and resentful at the con job that is monogamous marriage, and all of the cultural fairytale bullshit that goes along with it.
I know I am polyamorous because my heart breaks at not following through on my feelings, and because I feel a sense of loss over what might have been.
I think that, if you are in a steady relationship with someone, and believe you are in love, and yet you find yourself developing strong feelings for others, while still being in love with your SO, then you might be polyamorous.
This is a different thing from "fear of commitment."