For websites: is it easy to navigate? If you're on dialup, does it take a long time to load? Does it have ads (popup or otherwise) that might confuse a newbie?

Is the source up to date? Look at publication dates for books and 'last edited' notes on websites - for most computer stuff you don't want anything more than a couple years old.

Is the writing style likely to put many people off, either by being excessively dessicated or just too cute for words? This matters, even if you like it.

What previous knowledge does the author assume? If it's anything beyond very basic computer skills, you need to screen potential students and steer them towards appropriate background courses when necessary. Add some questions to your registration form.

What programs need to be installed to do the exercises? If you're teaching people on a public computer, all necessary programs should be installed, configured, and accessible from the desktop or Quick Launch. If people will be working on private computers, they'll need download links and detailed instructions.