Simple answer: There's no way to detect modifier keys from within an alias.
If the person has to call the alias with the modifier key anyway why not just use a different alias or an additional switch/parameter instead? If it's for the ease-of-use thing you could use the specialised F-key aliases so that you can then detect F<n>, Ctrl+F<n>, and Shift+F<n> by which alias is called (/f<n>, /cf<n>, /sf<n> respectively).
