It's quite a sight isn't it? Just before I first saw it, I wondered how I would pick it out (i.e what it would look like, how bright, how fast) thinking I might get it confused with an aircraft. However, you just know it when you see it for the first time. It's usually much brighter than anything in the night sky, save maybe Venus and moves too quickly across the sky to be an aircraft (and an aircraft has blinking lights and the ISS is constantly illuminated by the sun). It's very graceful and fills me with awe everytime
For realtime updates on the ISS and all space satellites, check out this site:
http://www.n2yo.com/