Science Fact: The filler words people use can predict—better than they can—if they'll be a good match, and if the relationship will last.
Explanation: You've probably noticed how groups of friends tend to talk more like each other after a while. The first time you heard somebody say "totes adorbs," you probably wanted to totes shove her off a bridge, but then later you found yourself saying "totes adorbs" all the dang time, even in response to things like "Who invented the cotton gin?" This is one aspect of something called language style matching, or LSM, and it is totes adorbs.
As far as dating predictions go, LSM is most important when you're using function words, which are basically the background words we all kind of ignore—prepositions, conjunctions, auxiliary verbs, and all of the other boring words. In the sentence "There are FIVE SHARKS behind you," your brain sort of skips over "there are" and fixates on the part about FIVE SHARKS. Those are the content words, and the rest are the function words, and they can predict relationship success better than a certified dating wizard.
The Science: Back in the '90s, a psychologist studying language built a program to analyze people's use of all these function words, especially in a dating context. He found that when people used function words in the same way and at the same rate, they were more likely to get along in a relationship. If those patterns grew more similar as they dated, they were more likely to stay together down the line. They don't realize they're doing it, but people who are interested in each other start to talk more like each other, and these meaningless, forgettable words are better predictors than the people themselves.
This kind of makes sense, because ordinary humans are constantly mimicking each other, unless they hate each other or are super-creepy. If other people yawn, you yawn; if everyone in the room keeps saying "wibble" for some reason, sooner or later you will start saying "wibble," because your brain is quietly thinking, "Do what everyone else does, or they'll kick us out of society and we will have to live in an abandoned cave."
So What Should I Do About It?
Well, you can't very well keep track of every single word some random girl is using. Or you could, but then soon she would be using zero words, because she would be gone, and you would be sitting there by yourself making tick marks on a napkin. It took a scientist and a computer to do the math on this stuff in the first place, and you probably aren't a scientist or a computer, so don't concentrate on that part. Instead, just be on the lookout for the general feel of mimicry when you're talking to someone.
If someone's into you, she will—very subtly—talk and act more like you do (and you'll do the same). One study found that mimicry increases with romantic interest; if you stretch and she stretches, it's a better sign than if you stretch and she steals your wallet. You can't notice every tine clue, but they're all there, in her body language and her language language, if you just know where to look.