I'm presuming you know how to prepare the SN to enable it be tested, to confirm purity. There is a thread somewhere on here that explains if you don't (just search).
The test strips give a wide range, the drop test gives a more accurate reading. Search for a company like API and nitrite test. The kit costs about £9. It's very easy to follow the instructions - basically prepare the SN solution for testing, put the required amount into a test tube (that's supplied in the API kit) and then drip the supplied test solution in according to the instructions.
Edit - just to confirm, you are looking for API aquarium test kits, so they're easy to find on Amazon.