How to tell if Shift is being pressed in a MouseDown event?

Use the Control.ModifierKeys property to see what's pressed. For example:

    private void Form1_MouseClick(object sender, MouseEventArgs e) {
        if (Control.ModifierKeys == Keys.Control) {
            Console.WriteLine("Ctrl+Click");
        }
    }

Other modifiers are Keys.Alt and Keys.Shift. Find combinations with, say, (Keys.Control | Keys.Shift).

In C#, you can check using -

Keyboard.IsKeyDown(Key.LeftShift) or key.RightShift

http://msdn.microsoft.com/en-us/library/system.windows.input.keyboard.iskeydown.aspx

http://msdn.microsoft.com/en-us/library/system.windows.input.key.aspx