NEW YORK (MainStreet) — Financial infidelity is a growing cause of divorce in the United States. A recent report from CreditCards.com showed that as much as 6% of all married couples had one person hiding money or even entire accounts. That might not sound like much, but it's higher than one in twenty, meaning it's either taking place in your marriage or that of someone you know.

The same study, from CreditCards.com, found that a full quarter of all married people think it's O.K. to spend up to $500 without telling their spouse. With statistics like these, if you're noticing any strange financial behavior with your spouse, you might want to do a little detective work to see if you're a victim of financial infidelity.

Keep Your Eyes on the Mailbox

"Is your spouse always trying to beat you to the mailbox?" That's the first question Kathleen Connell, a partner with Boyd, Collar, Nolen and Tuggle in Atlanta would ask. She notes that in some relationships some people just get the mail, and that's normal. In other cases, your spouse might be trying to beat you to the mailbox because of a birthday present or something similar. So it's not necessarily a red flag. But if you start getting the sense that your spouse is trying to rush to the mailbox to beat you there every day, and there are other signs of financial infidelity, start digging around a little.

Pull Your Credit Reports Together

If you do suspect something, Connell says you should start pulling credit reports together. She further adds that "people should never underestimate the value of pulling your own credit report." In a worst-case scenario, you might have credit applied for in your name, or you might have been put down as a someone with signatory power. In any event, there's nothing suspicious or malicious about requesting that you and your spouse pull credit reports together. Resistance to this might be a sign that your partner is hiding something.

Don't Assume The Money Is Hidden Somewhere Obvious

Pam Friedman, a CFP who mostly works as a divorce financial analyst, says that in a lot of cases, the money isn't hidden in a bank account or on a credit card. It's hidden in more exotic places like stock options or benefits at work. "It's usually pretty high level," she says. The issue here is that, for whatever reason, a lot of people don't think of these kinds of deferred-compensation plans as being "our money." Friedman says that even when she's sitting in divorce discovery proceedings, people might be confused as to why they have to list such assets as joint assets for the purpose of settling a divorce.

But Don't Assume It's Not Somewhere Obvious

Friedman says that a lot of financial infidelity can also be found in the simplest of places -- withdrawals and deposits. In cases where one partner is more in charge of paying the bills than another, and more attentive to finances, it becomes far easier to hide money without doing anything to hide it. This is especially true with small business owners or the self-employed. "It's mostly down to withdrawals and deposits at that point," Friedman says. Even a transcript or a bank statement can reveal a lot in these cases. It sounds simple, but Friedman says she sees it all the time.

It's Probably Not Getting Spent on Big Things

One mistake a lot of people make is that they think their partner isn't hiding money, because he or she is not coming home with a new Porsche. Connell points out that, while your spouse might buy something like a couch, he's probably hiding the 20 trips he's making to Target racking up credit card debt. Put simply, most of the time, financial infidelity isn't one large purchase, it's lots of little ones. It goes hand-in-hand with compulsive spending and generally hints at much deeper issues with your spouse than just being spend.

Following the Bread Crumbs

"If you suspect something, let's go down that path," says Friedman. The first question is what it's going to cost to figure it out. However, there's one easy way to inquire after missing or hidden money or assets. Ask. Because if you're not going to get a straight answer out of your partner, even when you confront them with grounded suspicions and hard evidence, your marriage has a lot more trouble than just money.

Written by Nicholas Pell for MainStreet