I find myself viewing source to see if they are using CSS or tables for layout.
I find myself increasing text size a couple of notches to see what breaks (amazing how many beautiful CSS layouts break after just 1 notch increase in FF)
I even (sometimes) find myself checking HTML and CSS validation!
I too check sites occassionally, for all the above, mainly the text size increase cause I try so hard to make sure my sites can take a couple of text size increases before they fall apart.
I only ever really test sites that are posted in 'site test' type forums. People are asking for constructive criticism so I look for things they might not have picked up on. Sometimes, like Johnny Gulag says, I'll check out other designers' sites but I don't do it religiously with every one I look at.
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