You’re reading the newspaper, and your husband or wife could be cheating on you at this very moment.

Not possible, you think?

Of the 19,000 U.S. adults responding anonymously to a national survey about their sexual behavior between 1991 and 2004, 13 percent of women and 22 percent of men reported having a sexual partner other than their spouse while they were married, says Tom W. Smith, director of the General Social Survey at the National Opinion Research Center at the University of Chicago. Although the figures remained relatively stable for men throughout that time, Smith says the numbers for women fluctuated between 11 percent and 14 percent, indicating a “small but clear upward trend.�?/P>

So, how can you be so sure they are â€�?or aren’t?

Relationship and infidelity experts, private investigators, technology specialists and divorce attorneys say if you know the subtle and not-so-subtle signs to look for, they’ll point you to the answer.

You can put your five senses to work. Or you can shell out hundreds �?or thousands �?of dollars to hire a private detective. You also can invest in the newest high-tech products on the market �?computer spyware, electronic tracking devices, in-home evidence-gathering kits among them �?in an effort to catch cheating mates.

David Vitalli, a private investigator and chief executive of Tru-Test Forensic and Applied Sciences Corp. in Newburgh, N.Y., says his company recently began marketing a patented home evidence-collection kit that will help spouses detect with 100 percent accuracy whether their mates have been intimate with someone else.

The kit contains an ultraviolet light that will detect stains on your mate’s clothing that are normally impossible to see or feel. Protein and enzyme formulas included in the kit also will identify the presence of bodily fluids. And if you require further proof, you can mail specimens you’ve collected in an enclosed envelope to a laboratory for testing to determine whether they match your DNA, your mate’s â€�?or someone else’s. The kit costs $79.95 (877-362-9900 or www.trutestinc.com). Sending specimens for laboratory DNA testing will cost at least $500.

High-tech checking

Suspicious spouses also are using global positioning systems, or GPS, to track their mates�?whereabouts.

Larry Wasylin, vice president of sales and marketing for Magnolia Broadband of Bedminster, N.J., has seen it firsthand in recent months during business trips to Asia. In one instance, he says, he was dining at a restaurant when a colleague pulled out and stared at his cellphone.

“I said, ‘What are you doing?�?He said, ‘I’m looking to see where my wife is.�?She was picking up the kids from an after-school program. He said, ‘She’ll be home in about 30 minutes.�?They’re marketing it right now under the brand iKids,�?he adds. ‘The idea is, it allows parents to ensure the safety of their children. �?It’s not confined to children. People like to know where their spouses are.�?/P>

Cellphones that capture video can do the same thing, he says, allowing a private eye to tape your mate and then stream data to you.

Wasylin says he also saw reports about a new chip inserted into a cellphone to allow suspicious husbands and wives to listen in on their spouses�?calls without them knowing.

Some are using the images they’ve seen and conversations they’ve heard to confront the cheaters.

He says you can expect to see this technology hit U.S. markets in the not-so-distant future.

Use all your senses

To Ruth Houston, author of “Is He Cheating on You? â€�?829 Telltale Signs�?(Lifestyle Publications, $29.95), gizmos and gadgets won’t tell the whole story. For example, she says, GPS will tell you where they are but not what they’re doing or with whom. Computer spyware will tell you the content of the e-mails going back and forth, but there is information you still will not be able to detect, such as the seriousness of the relationship or the identity of the other person.

Even private investigators are limited by what you tell them. The more detailed information you can give them, the better.

“You don’t need a lot of gadgets,�?says Houston, who has been researching infidelity for more than a decade since discovering her ex cheated on her.

“You can find countless signs of infidelity using only your eyes, your ears and your personal knowledge of your mate. The key is knowing what to look for.�?/P>

That involves being tuned into your mate’s work habits, daily schedule, and likes and dislikes, Houston says.

“Then you can zero in on what’s happening. You will see changes across the board. There will be things you pick up in their conversation, personal hygiene, how they relate to you, personal behaviors, changes in all those areas,�?which she lists on www.infidelityadvice.com.

Some focus on obvious signs (lipstick on the collar, coming home late) and overlook the subtle clues, Houston says.

For instance, your spouse takes a sudden interest in things, like volunteering to take over paying the monthly bills â€�?a job you’ve been doing â€�?to give you, he or she says, a much-needed break.

“You say, ‘That’s nice,�?but maybe he doesn’t want you to see the bills and what he’s been spending his money on,�?Houston says.

Once you have proof

Don’t confront your spouse with only your suspicions, some say. Go with proof.

Even with that, some cheaters will never admit betrayal, says Mark Barondess, a Los Angeles attorney and author of the new book “What Were You Thinking?: $600-Per-Hour Legal Advice on Relationships, Marriage & Divorce�?(Phoenix, $25.95).

“They could be having sex right in front of their spouse and tell them, ‘It wasn’t me,�?�?he says.

“People will do and say anything they possibly can to avoid admitting they were caught cheating.�?/P>

When you confront your spouse about suspicions, pay close attention to his or her reactions, looking for anything that would be a break from the norm: a glitch in his body language or a change in the cadence or pitch of her voice, says Greg Hartley, a U.S. Army interrogator for 15 years who co-authored the new book “How to Spot a Liar: Why People Don’t Tell the Truth �?and How You Can Catch Them�?(Career Press, $14.99).

“This is what catches most liars: We can’t practice, rehearse or create enough details to sustain a lie. It’s the little details that break a story,�?Hartley says.

“You can ask, ‘Where were you at 2 this afternoon?�?I can lie and say, ‘I was at work.�?But if I ask you to give me a timeline of your day, the details will bite you.�?/P>

Hartley’s co-author, Maryann Karinch, also says it’s important to approach the conversation logically and calmly so that it doesn’t get ugly or out of control.

Ultimately, Karinch says, you have to ask yourself which outcome you want: To catch your spouse in a lie? To salvage your marriage? To get a big divorce settlement? To hear he or she is deeply sorry?

“If you want to save the marriage and you are genuinely distressed that the person is cheating on you, then you need to come directly into contact with this person about the facts of the matter and the emotions of the matter,�?she says.