Identifying the harmonics on FFT

[harmonics]

How do you accurately identify the fundamental harmonic if you get a peak of similar amplitudes at frequencies that are really close to each other.

And furthermore, once you identify the fundamental harmonic, are the second, third fourth harmonics just multiples of the fundamental frequency? Even if the amplitudes of these harmonics are lower than the amplitudes of nearby frequencies?

 

[rbj]

in the musical signal analysis sub-discipline, we use something called the Average Magnitude Difference Function (AMDF) or Average Squared Difference Function or autocorrelation to determine the fundamental frequency of a waveform