I was on a recent transcon staring out the window and listening to Ch9 when another UA jet came flying by in the opposite direction and a little below us. It looked really close. There was no mention of it on Ch 9, no evasive action taken by my plane, but man, the other plane looked really close (enough that I could tell it was a 757 and not a 767) If it wasn't for the fact that it went zooming by and that I didn't expect it, I probably could have read off its aircraft identification number (you know, NUA9AF or whatever)

Of course, I am no expert on this, but maybe someone can tell me ways of knowing when close is too close? Meaning, I know the requirement is 1000ft or something like that, but what does that look like when you're staring out the window?