Here's an image of the ISS taken from the Space Shuttle (I assume). It looks like it's passing over the Straits of Gibralter.
All that surface area reflects a lot of light when the ISS passes overhead just after sunset or just before sunrise when the observer on the ground is in the dark and the ISS is still seeing sunlight. It's easy to spot the ISS if you know when and where to look. Here's a NASA website which gives quick and easy directions.
If I select New York and Kingston (the city closest to my house), I get a number of possible ISS sightings over the next few weeks. Since I don't like getting up before dawn, I might wait until Saturday, October 15 when I can see the ISS at 7:39 pm. It will be visible for a couple of minutes in the southern sky first appearing 10 degrees above the horizon in the south-southwest and then disappearing in the south-southwest around 26 degrees above the horizon.
|Sat Oct 15/07:39 PM||10 above SSW||26 above SSE|
A better choice, however, would be on Monday the 17th at 7:18 pm when it will be visible for 4 minutes and will be higher in the sky (reaching a maximum altitude of 56 degrees.
|Mon Oct 17/07:18 PM||11 above SW||33 above E|
By the way, the altitude varies from 0 degrees at the horizon to 90 degrees at the zenith - the point directly above your head. How do you estimate altitude? Hold out your fist at arm's length - it covers about 10 degrees of the sky.
The ISS that night will be brighter than any of the stars, even brighter than Jupiter - the brightest thing in the current night sky other than the Moon.
So, if you've never spotted the ISS, go out and try to do it. It will surprise you in its brightness and you can wave at the 3 astronauts currently on board! Here's NASA's ISS page.