Custom query var rewriting with only variable

Answered thanks to Milo’s comment.

I wrote a small function to check if the query variable exists. Note that get_query_var('listen') didn’t work for me.

function is_listen() {    
    $vars = $GLOBALS['wp_query']->query_vars;

    if ( array_key_exists('listen', $vars) ) {
        return true;
    } else {
        return false;
    }       
}

Use it in a conditional like so:

if ( is_listen() ) {
    // Display listen tab
}