>What is the simpliest and most reliable method of detecting if a specific screen is already active in a given session to prevent mulitiple launches of a modeless screen?

Within an application:
In the INIT of the application I declare variables for all forms. E.g.

private goMainWindow, goOptionsWindow
store NULL to goMainForm, goOptionsForm

The call of a form takes this code:

if not isnull( goOptionsForm )
   *
   * other scenarios are possible of course
   with goOptionsForm
       if .windowstate = 1
           .windowstate = 0
       endif
       .show()
   endwith
else
   do form Options name goOptionsForm
endif

In the menu you can skip/disable an option by testing the variable, e.g. skip if not isnull( goOptionsForm )

There's also the problem of not wanting the user to start another instance of the whole application. If that's your problem, then please tell so.