How do you measure the partial (frequency) of a particular note?
Example: The 2nd partial of the note A3.

Partials can be calculated if you know the frequency of the note. In
the case of A3, the frequency is 220Hz. The partials are multiples of
the base frequency-- so the first partial is twice the base frequency,
or 440HZ (i.e. A4) and the second partial is three times the frequency
or 660Hz (i.e. E3 in just intonation, very close to E3 in even
temperament.)