Originally Posted by
Sageous
Good question! I'm guessing the answer to that is different for each person who consciously experiences the state. Was that a dodge? How about this, then:
If you are familiar with WILD, you will know that there is a "pause," a brief moment of absolute stillness, in between the last vestiges of waking life and the first of dreaming life. If you're doing WILD correctly (including making your attempt in the morning after several hours of sleep), that pause is extremely short, though you likely have experienced it. You may have ignored it, failed to notice it, or noticed it and just didn't care, simply waiting for it to pass in anticipation of things to come; but you likely experienced it.
At night, when you first go to sleep, that pause can be very long, and, if you are able to pay enough attention to it (which I highly recommend, of course), you'll find it is less empty than it seems at first glance: thoughts still stream, but far more slowly and palely than you're used to, and if you listen carefully, you might hear sounds or voices that always stay just out of range (or not; that last bit could just be me). Or you might experience something else altogether; but when your self-awareness enters delta intact, you will recognize the pause for what it is.
I believe that if you are able maintain self-awareness through the entire initial delta period, you will know that you are there, and there will be no confusion with HI or other pre-REM phenomena. Also keep in mind that it is possible to dream that you are in delta, but once you experience the real thing, you will be able to tell the difference.
That may sound vague, but as I said, I'm guessing that the answer to your question is truly different for each person who consciously experiences the state... the pause, however, should be the marker shared by all.