The most accurate way is to get a liquid test kit that includes those. The strips are sometimes cheaper, but they are far less accurate and the liquid one will last longer anyway.
I use to use the strip test...the api master is much better.. and FYI if you can get a infant medicen dropper, pharmist have the free ones behind the counter, makes getting 5ml of water for the tubes much easier
checking nitrates is improtant too. they are not necessarily bad for the fish, but nitrates arent the best either. a test kit will allow you to make sure that they dont get above 20