If you know enough about two points in your system, pipe characteristics, fluid properties, elevations, etc.... you should be able to calculate the velocity using the Mechanical Energy Balance equation. You should be able to find it in Perry's, McCabe Smith, or many other common engineering references. You could always take a shot at just Googling it too.
To actually measure it would require some sort of flow element like an orifice plate or a rotameter, or there are non-intrusive methods like clamp on ultrasonic devices.
Provided its a non-critical line and since its just water (assuming its not sour water, waste water, really high pressure or some other kind of nasty stream) you could also just disconnect it and use the "calibrated bucket" and stopwatch method.
