And neither can a teacher once you've finished your lesson and gone home.
And when you play something for him he's only going to see you for 1/2 hour while you're actually sitting in front of him.

I mean, if you're talking beginner guitar playing 101, then sure, go get yourself a school teacher who can show you what fingers to use and how to play basic chords and scales. If you want to take your playing to the next level, then that's where I come in. Some of the basics I'd expect before even accepting a student is the ability to play at least one minutes worth of some 'showcase' material that involves several intermediate to advanced techniques.

Check here:
http://www.schmange.com/SchmangeLesson1.txt

This is a text version of the first part of lesson #1. (If you can't at least do this, you're better off learning more basics from a beginner teacher.)
Most lessons involve three assignments similar to this. For instance, I'll record 1 minute worth of example riffs and ask you to copy them. I'll record certain scales or styles, give you tabs and diagrams etc... and your assignment is to emulate them. (from beginning to end with no errors...cleanly and properly recorded)
I listen to them carefully and critique you, as well as walk you through the entire process...one note at a time if necessary. You get a reason for learning them and a reason why I'm particularly giving it to you.
If you get it...and I can hear that you get it, we move on to the next part of your playing that needs work. While we're doing that I'm keeping track of your progress, reinforcing what you've learned and seeing what techniques are working for you. (ie, I've got every single MP3 and assignment you've ever done stored on my HD)
Gradually if you stick with it, the assignments will get harder and harder until you're playing is at a pro studio level or you don't need me anymore.