The earliest any of mine has ever had a first heat was 5 months old, the latest 13 months old, although on average they are between 9 & 11 months. Occassionally a girl can have a dose of "Vagininismuss" (sp?) prior to fully coming into heat, this is a stringy to pale blood coloured discharge but without the vulva becoming swollen, usually they are in heat within a couple of weeks of this. A reliable test for them being in heat is the "white tissue test", wait until they have come back in from going "potty" then dab the vulva with a white tissue, if you find blood and the vulva is looking swollen then they are in heat!! You may be quite surprised by how much they can swell up, when I had my first girl I worried that I would miss her coming into heat, my friends assured me that was not the case and they were right, it is pretty unmistakable! Other signs of being in heat are frequent washing of the vulva, although some can be quite lazy of this, another sign is they may lift or "cock" one of their back legs at the end of their potty "courtsy"!!
As Karlin has pointed out this is a time when your girl is at a higher risk of pyometra, a potentially life threatening illness, so if you have any worries about whether your girl is in heat or has an infection then visit your vet, even if is for nothing more than to put your mind at rest! Hope this is of help?