Given with their complete lack of interest in boys, much physical intimacy and every fight seems like it's coming from the point=-of-view of a frustrated domestic couple, it seems more than heavily implied the two are in a relationship.
Sam and Cat shared a bed in "#Pilot"
They shared a bed again in "#FavoriteShow", but in a wrestle style embrace, when Cat tries to wake Sam up.
Sam clearing Cat's tears in "#FavoriteShow".
At the end of the episode, they are shown doing a "butt bump".
Cat just loves to get into Sam's personal space, as this just happens every episode.
Sam and Cat briefly hold hands in "#MommaGoomer".
Sam and Cat were bumping butts again in "#RevengeOfTheBritBrats".
Sam was sitting very close to Cat while watching Celebrities Underwater.
At the end, they are seen walking together, laughing and smiling, in slow motion.
Sam even allowed Cat to hug her!
Sam and Cat's argument sounded like a married couple would do in "#PeezyB".
Sam: I'm out working all night and I come back and find you with another girl?
Cat: I have needs!
Sam licked Cat's ear for no apparent reason in "#SalmonCat".
In "#Twinfection", Melanie pays a visit to help her twin sister Sam prank Cat. At Bots, while having her hair all messy, she jumps on the table where Cat, Dice, Goomer and Sam sits, and forcefully kisses Cat...on the lips! Granted, this was done as a prank and not something romantic, but this is the most Les Yay we've seen from the show so far.
Though, Dan has struck same-sex kisses in his shows before, so this is slightly less surprising.
And in the bloopers for that episode, Jennette sort of licks Ariana's mouth.
Their favorite TV show appears to be about a family of cross-dressers.
The Love Dodecahedron in #KillerTunaJump involves some Hoyay ships, like Cade, Puckentine, and Frobbie (though this is played for laughs)