When you find yourself working with more text than you can fit into your InDesign text frame, the easiest way to see how much copy is overflowing is to open the Story Editor. (Edit > Edit in Story Editor)
In the Story Editor, a red line on the left side will show you how much text is overflowing your layout. If you decide to trim the text to fit your available space, you can edit your copy directly in this window.
Source: This tip comes from the June 2007 issue of Design Tools Monthly. Members of a Creativetechs support plan receive a subscription to Design Tools Monthly in their monthly care package.