If you Instagram every cocktail and basket of fries you order with this person, you need to have sex. No one Instagrams food they eat with people they don’t want to have sex with. In fact, if you tag each other on Facebook consistently when you’re out doing things together, even things as pointless as going to the post office, you’re basically wearing a massive I WANT TO HAVE SEX WITH YOU AND I WANT THE WORLD TO KNOW sign. It’s the ultimate declaration of modern romance.
If you keep poking each other on Facebook knowing full well that a) no one does that anymore, and b) everyone knows what that means, you need to have sex. Similarly, if they’re the only person you ever want to talk to on Facebook chat, which no one ever willingly uses, you need to have sex. If they post hilarious and timely memes on your wall, by all means have sex, but under no circumstances should you have sex with them if they keep writing “miss u” or “<3 <3 <3 <3” because it’s clear they obviously have some attachment issues. There’s a delicate balance.
If they favorite everything you tweet, you need to have sex. People only retweet things when they’re particularly funny or interesting, basically being like “This is cool enough to belong on my Twitter feed too, so why not,” but favoriting someone’s tweet is like saying “I love this thing you said, but it’d be kind of weird and too personal to put it out there for everyone to see so I’ll just save it here to read fifty times more later.” This doesn’t apply all the time, but favoriting is like internet pheromones: you just know.
If you interact in any way on Tumblr, you obviously need to have sex. But then again everyone on Tumblr just needs to have sex with each other. Just a massive internet orgy of political correctness and hipbones and love.
If they know about random nerdy things you’re obsessed with, that’s also a clear indicator that you need to have sex. For example, if a light starts flickering and they make a reference to Kant, you need to have sex. If you’re going to the store for flowers and they quote Mrs. Dalloway, you need to have sex (or not, if you hated that book, but probably have sex anyway just for the sharp pickup). If you blow out a candle and they start quoting Macbeth, have sex right then and there, they’ve earned it. You get the idea.
Also, if they’re the only person you consistently drunk text that is not your ex (but it’s possible that they could be your ex; if the English language hadn’t intended that sort of thing, those words wouldn’t rhyme, duh), you need to have sex. That’s just how it goes. Plus, if over half of your text messages contain various combinations of <3 and ;), stop technologically masturbating and have sex already. No one sends that many winky faces for no reason, unless they’re not from this country and just think it’s cute. But even then you can’t be too sure.
If they give you backrubs without complaining it’s because they want to have sex with you. Seriously — if you’re ever curious if someone wants to have sex, ask for a backrub. There’s a point at which even your significant other stops giving you backrubs. No one ever wants to give anyone a backrub unless they’re doing it for money or they want to have sex. And if the backrub lasts more than five minutes, that means they also deeply care for you as a person.
And if they go out of their way to bring you snacks, you absolutely need to have sex. Bonus if they bring you snacks when you’re hungover. Double bonus if you’re hungover together and they actually volunteer to get out of bed and get the snacks — forget the sex, it’s true love. Also, if they know your Starbucks order by heart without having to ask you, you need to have sex. If they know it changes by season, extra sex. And if they know the difference between a cappuccino and a macchiato, that’s a marriage situation. 
