Well the best way to know its done is to check it with a hydrometer, which you can get from your LHBS for just a few bux. With that and a wine thief you can can check a small sample from the fermenter. If your going for a sweeter hard cider, you put it in the fridge when it hits 1.020, which is called cold crashing (yeast go to sleep at low temp). If you are going for a really dry cider, you just let it go and dont worry about the hydrometer readings, because it will stop when it runs out of sugar.
Another way of checking it is to just get a turkey baster and take small samples every couple days and see how it tastes. Sanitze the whole baster including the bulb.
Im assuming you used straight cider w.o sugars? If so, then you will end up with a lower ABV product.Further exerimentation with sugars will change how it turns out. Its fun to experiment with small 1gal batches to see which you like. See the sticky on this forum, there is a TON of info on sugars and yeasts...
BTW, when this batch is done, dont throw out the yeast cake at the bottom. you can use this as a starter for V2.0 in your quest for ultimate cider
