There was a post this morning asking about how many people disable JavaScript. Then I began to wonder what techniques might be used to determine if the user has it disabled.
You can use a simple JS snippet to set the value of a hidden field. When posted back you know if JS was enabled or not.
Or you can try to open a popup window that you close rapidly (but that might be visible).
Also you have the NOSCRIPT tag that you can use to show text for browsers with JS disabled.