........ who they're sleeping with, ......
How do you work that one out?
A common PI trick to ferret out cheating spouses involves doing searches for credit cards in the name of one of the married people and a person not his or her spouse; apparently, many men who keep mistresses (and some women who keep lovers as well) will get a credit card in the name of the married man and the mistress, rather than the married man and wife, and use that credit card for expenses related to the affair, so as to keep the financial information away from the spouse.
Another way to do it is to trace movements of people using the credit card. Hotel reservations using a credit card when the spouse believes that the person is late at work or away on business or whatever, people checking into a hotel reserved in someone else's name using someone else's credit card, that sort of thing. From what I've been told, most PI sleuthing involving suspicions of affairs has less to do with following folks around with cameras and telephoto lenses than it does to do with wading through financial records.
Same is true of criminal investigations as well. Most (all?) FBI field agents have accounting backgrounds.
