What are some ways to detect the fundamental frequency in a signal using without using FFT?
Count the time interval between zero-crossings (if the waveform/SNR allows...).
Autocorrelation, average magnitude difference function and their variants (YIN...).
Estimating the parameters of a damped sinusoids model (MUSIC/ESPRIT).
Several instances of Goertzel algorithm, or more generally other filterbanks (band-pass, comb) - in case there is only a small number of candidate frequencies that you want to discriminate.