They can be seen with the naked eye as very bright objects moving quite swiftly from horizon to horizon. Since, in a good pass, you are seeing the reflection of the sun's rays off of the objects passing overhead, it is impossible to make out any detail without a telescope. Problem is, the whole episode only lasts a couple of minutes....these objects are in low-earth orbit and travel very quickly across the sky so it make is extremely difficult to train a telescope on them in order to obtain any specific detail. It's very cool to see nonetheless. I have never used a telescope to observe the ISS and I have watched it pass overhead many times. You're either into it or you're not....I'm definitely into it.