Although I do check my girls' rear end regularly, especially close to kidding time, it's only looking. I don't really get my nose close enough to smell...
As Tizaala says if there is a smell (and it wouldn't be a nice one) then it may be indicative of a dead/decomposing foetus inside. Usually would also be some dark brown discharge.
Changes in behaviour are much more indicative of imminent labour - they are just restless, not interested in food (usually, but not all of them
) and get up/lie down/scratch the ground. Also then their rear end is usually quite pink and puffed up and the udder is normally really full too. Some discharge for a few days beforehand (but not like described above) is normal.
Any of your girls have a smell??? If yes, may be needing investigation!