I am not you. You have your own answers. But I'll try to answer like me.
How do I *know* beyond a shadow of a doubt I'm poly?
Because I love hard, and I love who I love, and that's usually loving more than one.
Because when faced with this, I decided to just live it, and I was open about my want to have ethical non-exclusive relationships.
At the time I did not know the vocab words, but I was dating many and narrowed down to a "V" with me as the hinge. Then I struggled whether or not I was going to add a third partner, and was sorely temped, but told myself not
to go there because that would be fun for me but not fair to all
in terms of time management. So that was the one I deliberately let go and did not even ASK to date though the attraction was there and I think was mutual. Loving ethically sometimes means letting people go before they even begin, and doing it because you do love them.
How do I *know* I'm not just saying it because it's convenient (ie, I don't want to hurt someone?)
Does not compute to me. What are you trying to avoid and why?
I speak my truth. I'm not NOT going to speak my truth just because I'm afraid of the response or I'm worried about hurting feelings.
Sometimes feelings just get hurt! The thing is HOW you do it. For example, I can't think of a single break up situation that isn't going to to hurt. But letting another go gently and trying to minimize the hurt is one thing. SMASHING HEARTS TO PIECES and not giving a damn is another.
And sometimes being wired poly is hard, because it would be a whole lot easier NOT to love many and not have to deal in ethics all the time. Everyone has a polysaturation point too.
How do I know I'm just not simply a slut?
Like heartbreaker type lying person who sleep around for her own jollies and screw anyone else's feelings? I was not this.
Or like an honest swinger who wants casual recreational sex? I was not this.
Or like a confused person struggling with sexual identity as poly, but not having any kind of sex language besides "good" and "bad." Since this is outside the mainstream "good" then this must be "bad" ergo I must be "slut?" I was not this either.
I was this -- well, it's MY life. And I live it as I see fit, so long as I'm honest and true to myself and maintain some self respect and am not hurting anyone on purpose. I'm good to go. Screw what other people think! They do not live my life. I do.