The diddley bow is played slide-style—that is, the pitch of the string is changed by pressing a glass or metal object (a slide) against the string.  When playing slide on a new instrument, I rely on visual cues to get close to a note, and then on my ear to nail it.  For advanced techniques, such as harmonics, it is critical to have the positions marked.  Therefore, I mark the note positions for two octaves of a pentatonic scale (root, flat third, fourth, fifth, flat 7^th, octave, etc.) on my instruments.  I use harmonics to find these positions, so let's spend a moment to talk about what harmonics are. 

When the string on a diddley bow is struck, the vibration of the string is complex, consisting of a fundamental (or, first harmonic, which is the loudest tone of the open string) and several higher order harmonics (higher overtones).  If you touch the exact mid-point of the string very lightly with your finger and then strike the string with a stick, you should hear a chiming sound, one octave higher than the sound of the open string.  This tone is called the second harmonic, and you hear it because you have damped the fundamental tone of the open string, allowing the harmonic to sound.  If you move your finger very slightly along the string, the chiming sound disappears.  The place where the chiming sound is the loudest is the midpoint of the string, and this is the place you mark on the board.  Similarly, if you touch the string a third of the way from the nut or bridge, you get a softer, but even higher-pitched, chiming sound.  This is the third harmonic, and you hear it because you have damped the fundamental and second harmonic.  Touching the string at a point one quarter of the way from the nut or bridge is the still softer fourth harmonic, two octaves higher than the open string fundamental.  You will need to strike the string fairly hard to hear this one clearly, and with the type of wire we are using, this is about as many fundamentals as I can hear distinctly.  The harmonics get softer because the size (amplitude) of the vibration of each higher harmonic is smaller than the lower ones or the fundamental.  With this in mind, let's now find the locations for our position marks.
 

Step one:  I find the center of the string and make a big pencil mark at this position on the face of the instrument (the strongest harmonic is here).  On a guitar, this would be the equivalent of the 12 fret, one octave higher than the open string. 

Step two, I next find the places where the string length was divided into thirds (there are strong harmonics here as well) and mark these positions.  On a guitar, these would be the equivalent of the 7^th fret and the 19^th fret, a fifth and an octave and a fifth higher than the open string, respectively. 

Step three:  I find the places where the string length was divided in fourths and mark these positions (harmonics are achievable at these positions as well).  On a guitar, these would be equivalent to the 5^th and 24^th frets, a fourth and two octaves higher than the open string, respectively. 

Step four:  using a ruler or by eye, I mark the mid-point between the octave and the two octave positions (five-eighths of the string length from the nut).  This gives an octave and a fourth, equivalent to the 17^th fret of a guitar.  I use a slide to check this by ear. 

Step five:  using the slide, I find by ear and mark the positions equivalent to the 3^rd, 10^th, 15^th and 22^nd frets.  I mark these in pencil, because I find that, after playing a bit, I sometimes need to adjust some of the marks.  The position marks may be more visible if you use small pieces of blue masking tape over the pencil marks.