Determine the bottom of a movieclip

Hey all,

  I have a movieclip with a dynamic text box inside it that gets filled with content from an external source.  It scrolls up contstantly and is masked, this works fine.  I would like to know how to determine when the movieclip's bottom is beside/parallel to the mask movieclip's top.

Basically, I want to find out when one movieclip is beside another.

any help?

Thanks in advance!