How can I measure if there is water flowing in pipe or not?

 

With some sort of flow sensor - http://en.wikipedia.org/wiki/Flow_sensor.

 

Several that come to mind are of a commercial nature.

This is home built and uses a magnet and reed switch

http://www.overclockers.com/tips1114/

 

... and here's a commercial one with hall effect sensor made by Honeywell http://europe.hbc.honeywell.com/downloads/EN2R9029.PDF
... and another Wikipedia overview article http://en.wikipedia.org/wiki/Flow_measurement
... and here's a Yahoo group dedicated to sensors http://tech.groups.yahoo.com/group/sensorforum