When I WILD, I usually am conscious with my eyes closed and feel myself drifting off to sleep (e.g. hypnogogic imagery, I "lose" my hands and no longer feel them). Then, the next thing I know I find myself fully immersed within a dream (sometimes without sight at first).
^This is the only way I have done WILD (quite an accurate description, actually). Those are good questions because I don't understand either! It seems to me your mind has to be pretty far gone before SP kicks in and then, like you said, how do you know? I can only guess that with some people (many people it seems) their minds are still fully alert when it kicks in the way it is for you when you wake up from it and you're still in it but fully alert. I haven't even experienced that. I lie perfectly still when I wake up to remember my dreams but I've never, ever, woken up and felt that I was in SP. I feel so left out, lol.