I watched one of my mares for nearly a month - we had a routine 3 - 4 times a day of how I would check her, she fooled me many times, then the day it happened, she didn't look any different, except ... the insides of her back legs were wet... wth? She had started spraying milk down her legs. That's how I KNEW she was going to have it that night. And, she did. I have pics somewhere of Trixie, half in her mom, and trying to throw her front legs out to stand. WILD.
Every one is different, and every pregnancy & delivery is different. There are tons of things to look for, you may see some, you may see all or none at all. Just have to wait and watch, and cross your fingers.