how to know if someone is practicing polyamory?

I have been involved with a man for a year who said he was divorced
but lived in the same house with his ex. I believed this and I visited
his home and was around the ex. She is with another man who lives in
the home and my partner swears up and down he does not sleep with her
anymore. There seems to be nothing between them she is rather rude to
him and he works alot and also does everything around the house with
the kids ,etc.. Well I recently found out that they are not divorced
supposedly legally seperated which I don't believe. I openly asked him
if they have an open marriage he would not answer. I was thinking
maybe they are polyamorous but it seems like to me that he would have
been honest. I could deal I feel with the polyamory but not with not
being honest. He said it was because I would have ran away. Any ideas
how to approach the subject with him to get at the root of all this.
He says he is there for his children I don't deny that he is an
excellent father. I was just wondering if this sort of thing happens
in polyamory or if everyone is open.