I test polarity using a radionics device. I test my own and I test the subject. If either test as not having a strong correct polarity then this must be corrected first to insure correct testing results. There are many reasons why polarity 'goes out' or 'switches' or 'reverses' (all mean the same thing). Sometimes its as simple as dehydration and lack of minerals.
There are other ways (than the radionics machine) to test polarity and you can easily Google for more information. #1) so you clearly understand the problem and #2) so you find a way to test and correct it that works for you.
Here are two quick links to get you started.
For more details, please check the link: