In WILDs I never actually feel SP. I feel the onset, but never when I actually shift. However in DEILD, I feel a very distinct shift in feeling and consciousness. It's hard to explain. It kind of feels like my entire body entered an electric field (rub a ballon on your hair and then bring it near your body, that feeling) and then suddenly went back to normal.

As for checking, the only way to check would be to try and move. If you can't move then you're in SP. If you can move then either you didn't get there, or are in a dream.