Distinguish between opportunity save and opportunity close in JavaScript

Hi,

I have written some JavaScript that triggers on an opportunity's OnSave event, in a nutshell it checks for open activities on the opportunity and cancels the save if there are none.

It's working great except for the fact that closing an opportunity as won or lost also triggers the OnSave event. Using GetSaveMode returns a value of 1, which is exactly the same as if a user had clicked the save button.

How can I determine that it was the opportunity close button that was clicked and not the save button?

Thanks,

Robin