Try this,
$cart = VirtueMartCart::getCart();
if(sizeof($cart->products) > 0){
echo 'Cart have products';
}
else{
echo 'Your cart is empty';
}
for redirections you can use follows
$mainframe = JFactory::getApplication();
$mainframe->redirect('url','message','message type');//message type is message,warning,error etc
Hope its helps..