What would you do if someone came to you about your partner having another child?

Now mind you it has been 4/5 years since the child been born but im after requesting a DNA test and so has he but she didn't want a DNA test... NOW all sudden she wants one but I feel like she won't even do it. We have been on and off but when she got pregnant I was still in a relationship with him... its been playing on my mind this whole time I want to know the truth if he did cheat on me when I was pregnant/postpartum with our child.
What would you do if someone came to you about your partner having another child?
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