Conventional testing methods are used; white box and black box.
Testing the white box for you may mean that the application should be able to make it go into a specific state, then you can predict the expected result.
A black box may mean that you click on a page, and then consider the possible results. Repeat and rinse until you get enough coverage.
Another thing that we use is monitoring statistics for our service. Did we get the expected number of views on this page. We run a / b tests regularly, and I checked a / b tests with refactored code to make sure that nothing has changed before rolling over.
/ Allan
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