We just met, so I don’t know him that well, but he was honest from the beginning. He says he’s still married because of their son, and because his wife doesn’t want to get divorced right now. Apparently, South Korea has a fault-based divorce system. TIL.
Ask his wife.
If she is ok with it, he’s not cheating.
There really isn’t any other way to know.
Even then, you don’t really know. They might be on bad terms and the wife might want to screw him over, or she might be in denial, or anything else. I guess the best chance at an honest indicator would be her level of surprise, rather than actively being ok with it, although even that might also not be entirely reliable.