Entering values directly vs. a list box

Hello,
I have kind of an odd question - Is it possible to have Navision determine whether a value was typed into a textbox or if the value was selected from a look-up window.

I have a customer that will be using Item Cross-Reference numbers. There may be instances where the same cross-reference number may point to several different item records. I want the system to display the look up window if the user types in a cross-reference number for which there are several records in the item cross-reference table that have the same cross-reference number pointing to mulitple items.

BUT, if the user selects a specific record from the cross-reference list, I don't want the system to do anything except it's normal processing. I am aware that I can put code in the OnValidate trigger that will open the list box. BUT, won't that same code execute when I make a selection from the list box?

What I am trying to figure out is how I can tell Navision that the user made a choice from the list rather than directly typing into the textbox.

I'd like to do this at the table level instead of adding code to a form if at all possible.

Thanks for any help anyone can give me.