When right clicking while you're in creative the output of InventoryClickEvent is invalid.
As far as I can tell there is no way to detect if a player right clicked or not.

When executing the code below all of the values are exactly the same when you left and right click.
However, when you do it inside survival mode it works as intended.
I've also tried using InventoryCreativeEvent just in case but it has the same result.

@EventHandler
private void invClick(InventoryClickEvent event) {
        Bukkit.broadcastMessage("" + event.isRightClick()); /* false */
        Bukkit.broadcastMessage("" + event.getClick().isRightClick()); /* false */
        Bukkit.broadcastMessage(event.getClick().toString()); /* CREATIVE */
        Bukkit.broadcastMessage(event.getAction().toString()); /* PLACE_ALL */
}