When you know the number of samples and the sampling frequency, then you can assign a frequency to each fft bin. Pick out the fft bin that has the largest absolute magnitude, convert bin number to frequency, and you have a start at the problem.
But only a start, because the largest absolute value does not necessarily occur at the zero'th harmonic of the fundamental. You can do searches like looking to see if there is non-trivial energy one octave down, but there can be cases where the fundamental frequency is absent even though harmonics are present.
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