The main thing will be figuring out the DFT bin that the frequency of interest falls on. The frequencies are spaced at Fs/N where N is the length of the input signal and Fs is the sampling frequency. Keep in mind that the first bin (unless you use fftshift) is the zero frequency, or DC. I'll do an example assuming a sampling frequency of 1 and two frequencies, 1/8 cycles/sec and 1/4 cycles per second. For a signal of length 200, we expect these frequencies in bins, 26 and 51.
n = 0:199;
x = 10*cos(pi/4*n)+30*sin(pi/2*n);
xdft = fft(x);
xdft = xdft(1:length(x)/2+1)/length(x);
xdft(2:end-1) = 2*xdft(2:end-1);
plot(abs(xdft))
abs(xdft(26))
abs(xdft(51))
