Trap Modifier Key

I want to add functionality to one of my forms,

if a button is clicked while the user is holding down the Ctrl and the Shift
key, i want to display a bunch of debugging/trace information, as well as
performing the default action (which is already coded).

So my question is: how do i detect which modifier keys are held down in a
button click event handler?  Is there a trick to this, or am I going to have
to tie into KeyDown event and set a flag,  resetting the flag on KeyUp ?

Thanks