For me, this seems backwards and like a waste of time/energy. For one, if you just ask the question, you may find out there is nothing to investigate further. That there is a perfectly innocent explanation...
Secondly, just about every good investigation starts with bringing the suspect in and calmly asking the pertinent question(s). That’s how you know if they’re lying and hiding something, that’s how you know in what direction to go to build your case or investigate further. First line of defense should always be your own intuition and common sense, IMO. I want to look my husband in the eye and gauge his reaction. Asking the question doesn’t mean you can not still move stealthily after the fact. You don’t have to let on that you know he’s lying and you’re about to F his whole world up.
I don’t understand running out lining ducks up, consulting divorce attorneys, spending money on PI’s, when you haven’t asked your man a single solitary question. That doesn’t seem like the behavior of someone in a healthy relationship.