A quick search on line ...

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A quick search on line brings up a whole list of companies who are selling on-line nitrate sensors.  Never used one, and I am not sure how they work, but, presumably, some of them are reliable, and work in a drinking water context. 

With respect to laboratory testing, as others have noted, there are some pretty decent kits around, for example, the Merck Kit.  If you are thinking about sending the samples to an external accredited laboratory, it might be worth looking at the standard methods for nitrate analysis detailed in the current edition of APHA's Standard Methods for the Examination of Water and Wastewater, and find a laboratory that offers one of the recommended methods.  APHA's text is still the definitive source of recommended standard methods for the water analysis.

Apologies for not being able to be more specific on the details.

Cheers

David Sheehan