Quite simply do NOT test the water at all. Why?
Because you can get yourself into heck of a muddle once you start testing for the following reasons:

A) Reagents in test kits normally have a short shelf life
after which they can produce very misleading results.

B) Many test reagents are inaccurate anyway and give misleading results.

C) Important parameters such as pH, ammonia and nitrites within the pond itself change during the day.
Test results thereof will also change depending on time
of day and when you fed the fish for example. In good faith based upon a test you add a "correctant" to your pond to find on a later test it has not helped. You therefore add more or add a "counter-correctant" Eventually you go around in circles and actually create a real problem by virtue of what has been added

D) In my opinion the ONLY time you need to test is when you believe you do have a serious problem and need expert and immediate action. This type of action cannot be catered for by using a low cost test kit with dubious accuracy. The best test kit by far is the behaviour of your fish.

If you must test and after all this is a great hobby then get the best test kit you can find .... Like many things price is probably the best determinant of quality.