<input type="checkbox" id="something" />
$("#something").click( function(){
if( $(this).is(':checked') ) alert("checked");
});
Edit: Doing this will not catch when the checkbox changes for other reasons than a click, like using the keyboard. To avoid this problem, listen to change
instead of click
.
For checking/unchecking programmatically, take a look at Why isn't my checkbox change event triggered?