The moment you call your new girlfriend by your ex-girlfriend's name.
The moment you accidentally wear your girlfriend's undies to work because it was dark when I got dressed.
The moment you find out you're related to your girlfriend.
I worked with a bloke who had this experience. His fiance was over for a family dinner and they were looking through the photo albums (we kept photos in big books back then kiddies) when she suddenly pointed at photo of an old woman and said, "That's my grandma!"
"It can't be," he told her. "That's my great-grandma."
"I think I know what my own grandma looks like. I'm telling you that woman with the hairy mole and a stubby is definitely her."
And it was. It sure made for some confusion at their wedding, what with people having to decide which side of the church to sit on. But they've got three kids now and none of them have two heads, so all's well that ends well.
Which brings me to an interesting phone call I had tonight.
"Dad," said Master20, a hint of panic evident in his tone. "You know how Grandma's maiden name was West? Well, West is a common name, isn't it?"
Seems Master20 and his girlfriend are discovering they have a lot more in common than they suspected - in this case, similarly named Grandmas.
You know you've found your soul mate when they immediately feel like family. Right?