Soulmates share dreams. You can't consciously control it (because dreams), but a life shared like that means that you'll recognise your soulmate on some level when you meet, though it can take a little while to confirm. Sleeping in physical contact (ie, spooning) solidifies the connection so that you can share thoughts while awake.

Request - http://tfa-kink.dreamwidth.org/3467.html?thread=6381451#cmt6381451
Only in Our Dreams - http://tfa-kink.dreamwidth.org/3467.html?thread=8057995#cmt8057995