I don’t think you can really get them to admit it unless they want to.
My one will occasionally admit it in like an addiction sense when I am not there because he starts thinking in depth about everything. He recently said he has an addiction problem so via text so I was able to screenshot it in case I need it in the future.
Otherwise, he openly is doing this in front of me so he knows I know. At the same time, he’s tried to hide the financial side from me so I don’t know the extent.
If you need proof for something like child residency because he’s pursuing that, then you would have to go to family court and raise your concerns. They have the ability to order him to be tested.