You have to be able to PROVE it.
You have to hire a private investigator to show the lifestyle she leads exceeds her income and can only be made up with your support.
Then you go in front of the judge to get an order to have a FORENSIC accountant go over her books with a fine tooth to see exactly where the dollars went.
It IS an expensive proposition to prove a point. It is generally pursued if you are looking to change custody for cause.
I have a friend who had it easy. His wife filed for divorce on claims of abandonment while he was serving in the Army overseas (she opted not go go with), and she didn't tell him about it. He came back to the US just in time to find out about it, get in his Class As (nice touch) and collect the orders relating to the change of station, and walk into the court during the hearing where she was about to get everything.
The judge was ready to have her hauled away, but he was nice for the kid's sake. She got only what he was willing to give and only on his terms, such as she has to show reimbursable receipts for all expenses just to get child support (he didn't think a single dad in the Army was a good thing for a kid, and I don't blame him).