how to calculate flow rate through a cross-section

This should be easy, but I've been trying for a half hour now and can't seem to do it.  I've simulated a flow through a series of tubes which branch from two inlets to multiple exits.  Using the surface parameters I can calculate the mass/volumetric flow rate at the inlet and exit faces.  I want to get those values at a few intermediate cross-sections along the length of the tube, but I can't figure out what face to select.  I've tried creating planes at the positions, but they don't work.  I've also tried selecting the tube cross section (I can select the tube face in a few places where tubes butt up against each other) but that also doesn't work.

Help!  I need this info for a report due this afternoon...

Michael