The only signal I had was that food suddenly tasted weird. I had a strange 'metallic' taste in my mouth and for some reason, all meat tasted 'off' to me.
Oh, and cigarettes went from smelling pretty ick, to smelling DIVINE. I loved it (I mean obviously I didn't because I was trying to quit for the sake of bub's health, but holy moley every time DH lit up outside I was melting all over the place and hanging my head out the door to get a bigger whiff hahaha).