Hi,
I have a very nice sounding whistle, but it is sadly out of tune with itself. At least according to some tuners on the iPhone, one of them being RTTATuner.
I am not sure if the iPhone the most reliable way of checking the tuning.
What is the best method of checking if a whistle is in tune (with itself)?
I think it is wise to double check before ruining the body 
It all depends on how developed your ear is. Some people have Perfect Pitch and have no need for tuners of any sort.
Other people donât have Perfect Pitch but have a keen ear for Relative Pitch. All good pipers are like that.
If you have good Relative Pitch, or want to practice developing Relative Pitch, I think the best way is to get a tuner than generates tones. For a D whistle have the tone generator play a D, and practice playing various long notes on the whistle, adjusting your blowing on each note so it blends with the D. Do this long enough, and youâll both develop excellent Relative Pitch, and learn how to blow your particular whistle in tune.
But there are people who donât have a developed ear at all, and must rely (at least for the moment) on watching a needle. For that, get a tuning app that allows you to select Just Intonation. But beware! Oftentimes Just Intonation assumes the key of C. Thatâs fine if youâre playing a C whistle. But you have to be able to adjust WHICH key the Just Intonation is calibrated to for whistles other than C. You want to calibrate to D for a D whistle, and so forth.
Listening. (Seriously!)
Awhile back on one of our groups Monday nighters we had four or five various types of tuners, cheap & expensive, internet, smart phone apps, etc., and none could get everyone in tune and we just listened⌠as Peter just stated.
All of this technology. 
The RTTA tuner is quite clever. You can play ânormallyâ, and see how off your whistle is (or you are!). If you watch the needle on a regular tuner, you have a tendency to blow it into tune. Sometimes that is what you want, but the RTTA is more true to how you actually play.
Some whistles are more in tune with themselves than others. And some notes are more difficult than others for the manner to get right.
With the RTTA tuner, I watch to make sure the louder and more shrill notes in the high register are in tune, as that is where you session mates will be more easily offended.
âJust listeningâ works if you have Perfect Pitch, or well-developed Relative Pitch.
Oftentimes people who have these things assume that everyone else has them too.
For those that donât, theyâre skills which can be developed, an effective proven method given above.
Yes Iâve never seen a tuner that could get people in tune! A tuner only tells you if youâre in tune, or if youâre flat, or sharp, and by how much.
Itâs up to the player to do the tuning.
It works if youâre musical enough to be playing an instrument (that doesnât produce absolutely fixed pitches) at all⌠which includes all woodwinds and brass as well as the more obvious bowed strings and trombone with no clear âmarkersâ for pitch positionsâŚ
The question was âWhat is the best method of checking if a whistle is in tune (with itself)?â, not âin tune with some predefined standardâ.
I like the Flutini tuner (free download). What I do with it is play a few tunes with it without looking at it. It stores and averages out my playing so when I look at it I can see how in or out of tune my whistle is, and can adjust the size of the holes as needed. My thinking is as you are playing different tunes at different tempos you will naturally be, intentionally or not, using a different amount of force, so tuning your holes for your average playing is most likely to allow you to get closer to where you want to be at any given time.
One thing that is overlooked about tuning, is blowing the correct pressure for your instrument.
Try playing someoneâs whistle who sounds great. You may have to blow up or down to suit it.
Every whistle has its needs.
I agree that with massed produced whistles you may have to alter your breath pressure to get the whistle to play in tune, so if you have the opportunity to tune your own holes I believe you can get closer to what is right for you. And yet the Goldie low Dâs Iâve played are very close to in tune for me playing them without even thinking about the tuning.
Precise tuning is a somewhat relative term when applied to a whistle and it depends a lot on who is playing the whistle. Differing breath pressure can make a huge difference. Try playing up and down the scale in thirds and see if that sounds OK. Even better-- go play with other players. If your whistle sounds good with others, itâs well enough in tune for all practical purposes. You can drive yourself crazy trying for perfect or precise.