FINDING NOTES ON THE UNFRETTED NECK

With an unfretted string instrument, you can put your fingers anyplace you want on the strings and the result may or may not be a note in western music that is available on a keyboard. 

To play in tune, you  can try several things that should help:

Learn the chromatic scale like the back of your hand, as follows:

C
 C#(=D-flat)
D
D#(=E-flat)
E
F
F#(=G-flat)
G
G#(=A-flat)
A
A#(=B-flat)
B
C again.

I've tried to think of an analogy.  Finding these notes is pretty easy if you have perfect pitch.  It's still somewhat easy if you have good relative pitch so you can compare the note you are playing to notes you already played.  But if you know the chromatic scale, and if you know what the open strings are, you can use it to navigate your way up the string.  If you don't know the chromatic scale, it's like an airplane pilot trying to fly at night over unlit terrain in a fog and with no compass, no GPS, and under a cloudy sky and with no map.  At least, if you know what the open strings are and know the chromatic scale, you have towns illuminated and you have your map.

Also, as you are training your ear, you can work with a keyboard, even a used one off of ebay or somewhere, so long as it plays in tune.  Play the note you want on the keyboard (or have someone else do that) and then find the note on your instrument.

You can also compare the note you are playing with other open strings on your instrument, assuming it's in tune.  On a violin, if you are trying to play a D, E, G, or A, you can compare that to one of the open strings on the instrument. 

If you know how to hear when intervals are in tune (like hearing a perfect fifth, or perfect fourth) then you can make comparisons there also, if you know that the note you are wanting to play is a perfect fifth or fourth away from one of the open strings on your instrument.

So, basically, you are not lost without frets, but you need to find ways to navigate throuth the fog, overcast, poor lighting, and lack of any GPS.

Note.... a good chromatic electronic tuner for $30 makes a great musical GPS.  Play a note into the tuner and it will tell you what note you are playing and how far off pitch, low or high, you currently are.

Also, get your hands on the fretted equivalent of your unfretted instrument for awhile, even if it's just to hold it for ten minutes at the music store.  They won't mind too much, because you might buy one there sometime.  Find out how far your fingers need to be to play consecutive frets at various points along the neck.  And on short necked instruments, check the same thing for two-fret sintervals.

Instrument Tuning and Note Location