Let us establish straight away what 'in tune' is - it is when the interval (distance) between two notes is the same as one which occurs in the natural series. How do you know, if you haven't the benefit of the modern physicist's sophisticated gadgetry? The same as musicians have always done - you listen! You can hear, or feel even, when two notes are out of tune, 'disharmonic', because they jar, you feel uncomfortable, you experience an unpleasant vibration or a buzzing noise, the two notes "beat" against one another.

Roger North (c.1700):

"in tuning 2 [organj pipes to an octave there shall be first (when wholly discordant) an intollerable jarr, and as the artist moves the plugg or plate to reduce the sound to a consonance, first it shall come to a distinguisht beating very swift and chattering, and so slower, and at length to a sort of wallowing of the sound, and at last fall into the consonance, as into a notch, and there flow with all the quiet and eveness imaginable." (p. 204)

The jarring is especially noticeable when children play recorders together: and indeed, of all the intervals used in 'tuning' a modern piano, only one, the octave, is actually in tune. As you reach the point where the "disorder..., chattering, or beating... dyes in the accord" (RN 207), you experience a feeling of relief, your whole body tells you it's all right now, that they're in tune -always assuming, of course, that you're free of traffic noise, humming fridges and other aural polution.

With a bit of practice, and if you have a clear idea of what you're listening for, you can play in tune with any instrument on which you can vary the pitch, and you can sing in tune too, of course: but the situation is different with instruments which cannot readily be adjusted, like those with fixed frets and keyboard instruments.