Is my partner having an affair? How YOU can find out.

Private Investigation, "Gills" story.

It has to be one of the worst things imaginable in any relationship, discovering your husband, wife or partner has been unfaithful.

If you're not sure, there are many things that you can do to find out the truth. We sat down with D Shaw, Senior Investigator at Watkins Ward, UK Private Investigator who gave away a few tricks of his trade, to catch a cheater.

Our victim "Gill" explained to us what it was like when she discovered her husband had been having an affair behind her back.

"I did not want to believe it. It's....it's just a fog of total disbelief. I put some guilt on myself. It's so painful. I just wanted to believe nothing was happening, until I just had to."

"Life shattering", "Gill" added.

It's the life-shattering moment that takes its victims by surprise: Finding out your spouse has betrayed you.

That's exactly what happened to "Gill", who asked that we conceal her identity. Eight years into her marriage, she began to notice changes.

"He was so clearly distant. Very little conversation. Lots of places to go during the day", Gill told us.

Her relationship was dissolving. He found someone else. But how did she know? In fact, how does anyone know if their significant other, is no longer significant?

Private Investigator, Daniel Shaw told us "If your gut is telling you something is not right, 99 percent of the time it's right."

Shaw has been a Private Investigator for 17 years. When the possibility of infidelity is on the table, he says there are many red flags to watch out for.

"The physical changes are one of the most obvious. Caring all the time about how they dress," Shaw said "The spouse is coming home late. They are taking their phone with them, everywhere. The code is locked or different. Face down. On silent."

Shaw says that smartphone could be the key that tells all. Are they overly protective by sleeping with it under the pillow? Can you see what's on their phone? Are there dating apps, or messaging apps like Snapchat that delete messages once they're read? That could suggest intent to find something outside of the marriage.

Another warning sign?

"They haven't been having sexual relations in years and suddenly there's a bottle of Viagra," Shaw told us.

He suggests hiring a professional because they have the tools to legally collect evidence.

Shaw said, "Carter-Harris Private Investigations, the company I work for is ICO registered for Private Investigation services in respect of the way we gather and handle data."

"There are a number of tools available to us including, for example; professional online forensic analysis and detailed background profiling"

Now if you want to go about this on your own, you have options.

"You can go to the on-line spy stores or big e-tailers and there's hundreds of things you can get" Shaw said.

But it doesn't mean they're legal, so you want to make sure the evidence collected wasn't obtained illegally, so its admissible in court.

"There's a lot of different ways that people can get caught, and they do get caught. And that's where we step in," Shaw told us.

As for Gill, she's moved on since her divorce.

"It can be a great thing to get rid of someone so toxic in your life that you're living with", Gill said.

Something to keep in mind, there are many products on the market that allow you track or record a suspected loved one's actions. It's not illegal to buy them, but it could be illegal based on the way you use them. Before going about this alone, experts say it's best to consult with a professional Private Investigator or Solicitor.