One sure fire way to know that you are pregnant is when your sense of smell kicks into high gear. Things that didn't bother you before may now cause you to gag.
I used to smell things five minutes before people I was with would. For the good things in life like flowers and food, it was a treat. But when your mate comes to bed after a few beers, you'll start building a pillow wall between the two of you just to stay sober.
Unfortunately, it lasts for the duration of pregnancy and into mommyhood. Then, you'll be back to your old self and wish you had that sensitive sniffer back to eek out that last bit of baby's freshly shampooed hair.