You could know it if you payed attention to the scenes
and if that alone wasn't enough you had the script, the novelization and some of the comics and you could read what the writers said. You'd understand then that these two characters were in fact a couple way before his planet got destroyed and this is the very reason why their interactions in the turbolift and later on the transport pad, make sense. He could act like that only with someone who was his girlfriend.
Besides, if one can buy that two people that despised each other can suddenly become besta friendas forevar only because dude's future self from another reality played matchmaker and they can convincingly play a ala TWOK scene with a crying, screaming and homicidal Spock acting like that for the death of someone he hardly considered more than a co-worker for the majority of the last movies (when he didn't even cry when his mother died and his home planet got sucked in a black hole and he felt his people dying)
.... then him having a girlfriend who happens to be someone that he probably knows since years shouldn't be that much of a shock or considered that impossible and/or OOC.
