Try an ultrasonic flow meter.  Basically you aim a 40 KHz tx/rx at the
pipe and if there is a phase shift, you have flow.  It gets a lot more
fancy if you want to know the exact flow, but for your application it
should work well.  For more information, google ultrasonic flow meter.
 I love the idea since it's non-invasive.

This was the reason why I was searching for ultrasonic pairs about a
week ago.  :)

Hope this helps,
Aaron Tufgar

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