My little girl just finished her first (and last) season and she gave us no indications what so ever, she acted no differently and it took us unawares, but then I was a complete novice to the whole scenario so didn't know what I was looking for anyway!!
We have a "heat" barometer over here. My 3 boys thought Grace smelled LOVELY. She was a month earlier than expected too. But I gotta trust those noses!!! They knew about a week ahead of time!
And for as pathetic and gross it may seem, dog owners need to spend some time looking at their dogs' butts. One day Grace just seemed a tiny bit swollen and there was a drop of red pee, and sure enough, she was in season a month early.
My girls either have too much coat on their rears or I'm not spending enough time starring, because I don't notice any outward signs of them coming in. I do notice the boys sniffing, which starts a couple of weeks ahead of time.
one sure thing is that they start sort of "marking" like males, leaving little spots here and there during their walks. All of my females have always done this for up to a couple of weeks before I start seeing the red spots.
Male dogs also start getting interested (most of them want to play and make friends, not necessarily mount)
Some girls become more aggressive with other females also