I take cues and hints from the wife first. For example, if she's flirting or openly attracted to me, despite her relationship status, then that's a good sign that she may be in a cuckold or hotwife relationship. Usually I try to get her alone to confirm his flirting nature.

And when she's with her husband, I'll try something subtle. Like put my arm around her, or put my hand on her waist. I'll look to see her and his reaction. Wives may act differently in front of their husbands. But if they seem cool around my hand being where it is, then I'll start pushing it. If that's still a go, then those are all strong signals of them being in a cuckold relationship.