NEVER press the test button first,
this is the last test you carry out, and only if all other tests are satisfactory,
the reason is that the RCD could have 'grown' together, and therefore would not trip in case of a fault, which will show on your tests,
pressing the button will usually mechanically force the RCD to open, thus now allowing it to trip on your tests,
but this would not happen in the real world fault scenario, you have artificially forced the RCD to work by using the button.
also, if on your first test the RCD fails, DONT test it again looking for a better result, the poor chap on the other end of a live cable wont get another chance