Check Zandors 'When to harvest" post in the growfaqs forum. You want to look at the Trichomes and the color/clarity of them.. The cheapest way to do this is with one of the cheapo little pocket microscopes from Radio Shack (Part number 63-1133 )

In my avatar, this bud had about ~ 1 week to go.. and the pic is taken through that cheesy little microscope!