What is most accurate PH test

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Hi Guys,
I am new to this forum but have been growing orchids for a few years now and as I live downunder I have got quite addicted to Aussie Natives.
Since I have been growing orchids I have got to know that PH is pretty important but have not yet found out the most accurate way of testing it,I have bought a cheap PH test probe but to me it seems to read the same all the time so I have my doubts about it being any good.
So the question is, what is the best method of testing.?

PH meter.?
Solution.?
Litmus Paper ?

Whichever one, can you recommend anything as I can order anywhere around the world over the net I presume.

I would appreciate any information you guys can give me and how do you preform the test,do you drench the plant and then test the run off water,which would take a long time with a few hundred plants or do you just test a few random ones to get an average.
Thank you guys in advance.
P.S Pardon my ignorance but what is RO water.