I have said this before but I think asking how to know if one is poly or not could be a waste of time. The more important questions, as I see it anyway, are:
What kinds of relationships do I want in my life?
How do I go about creating them?
What brings me fulfillment in relationships?
What effect do I have on the people I love and care about?
How can I live my life so that my relationships are the best possible expression of who I am and how I love?
Compared to those, I think "Am I poly?" is like asking "Am I blonde?" No matter the answer, it doesn't really amount to much without also asking some of the above.