How to tell an element is visible with jQuery

To check if an element is visible
if( $('#foo').is(':visible') ) {
    // it's visible, do something
}
else {
    // it's not visible so do something else
}

To check if an element is hidden
if( $('#foo').is(':hidden') ) {
    // it's hidden, do something
}
else {
    // it's not hidden so do something else
}