The first sign is when you start wearing your 6"4' husband's T-shirts featuring graphics of a corn dog on them to parent-teacher conference, because that's the classiest thing you own that covers your mid-section.
That's a pretty good indication you're getting Mondo Preggo.
But its not til you wear your GRANDMOTHER'S clothing that you're Uber-Pregs. When you wear the ankle-length, long-sleeved white polyester nightgown your grandmother bought at the 99 cent store, that's when you know your water's gonna break soon. Particularly if you wear that nightgown to bed as a means of seducing your husband, the sole motivation being that you're trying to induce yourself into labor. The grossness on so many levels will indicate to you that you'e 40 stinking weeks preggie and about to enter a whole other realm of grossness, an alternate reality of physical grostequerie known as childbirth and post partum.
And that's where I am right now folks. Wish me luck.