The set cookie-and-read-it-back approach is by far the best one since it explicitly only uses the feature you are trying to test. If you used JavaScript to do it, and JS was disabled, then you might return a false error that cookies were disabled.
Of course dynamic NT pages depend on both cookies and JavaScript to work - but it's probably better if you test for both, and error on the _correct_ one.
I'm not so sure about using a timer - I'm allergic to timers most of the time.
Cheers
Bruce